Attempts to try are...shall we say, extremely unwise. If you somehow manage to freeze time in the moment between one state and the other, you'll find the rider engulfed in a MASSIVE energy field. (I actually display this in the art.) If you're outside said field and try to enter, or you are inside the field as it's created, all said energy, even with time frozen, is enough to instantly vaporize you.
Now, normally, this doesn't happen: if time is not frozen, then if you're holding onto a rider as they transform, the transformation is so quick that the energy field doesn't vaporize you. (It will, however, probably break your grip on the rider, allowing them to be free.) It's the attempt to stop the transformation that produces the vaporizes-you-in-your-idiocy effect.
For proof of this, you can refer to Lord Darkblood. Now, he wasn't trying to stop a transformation. There was also no time stopping. So, if it were a normal transformation of a rider, then he would have actually survived. But Lord Darkblood suffered the fate of being vaporized because it was Ruby's first transformation, which unleashed all her power in one long moment, a moment long enough that the energy field existed for more than the split second, ergo, Lord Darkblood was toast. So, yes, normally, vaporization is only possible in the moment a rider becomes a rider.
...But if you're stupid enough to freeze time mid-transformation while holding the rider, you'll quickly find in the moments before your cessation of existence on the mortal plane that you'll share his fate. So, to keep a long story short, if you're a prospecting villain who sees a rider about to transform and you have thoughts of stopping them: don't. It's possible to stop many forms of magical girls from transforming, but riders are not among them.
In short: transformations from a rider can't be stopped by any method once started. So, don't even bother trying. However, note the wording. It is possible to prevent them from transforming in the first place, and/or force them to un-transform. For a start: while all riders with enough experience can gain the ability to transform on simple thought, almost all riders start out with the need to go through some sort of ritual to transform: motions of their arms, shouting a particular phrase, and similar cliches. Interrupt that, and you stop them.
Another pragmatic solution is, prior to their transformation, just knock them out or even shoot them. They can't transform if unconscious, or if dead. Their rider powers/reflexes post-transformation will probably mean shooting them becomes less viable. Heck, riders who are strong enough may be able to prevent themselves from being taken out by this in the first place.
But one of the easiest, most surefire ways to nullify the rider is most forms of anti-magic. These can be difficult to obtain/maintain, which is why most only exist as a brief defense in, say, a room of critical importance. But if you can lure a rider into one, they can't stop you. (Their spirit totem will still function, and will warn them of the impending anti-magic if the anti-magic is not well-hidden, but if they're in there, doesn't matter.)
Admittedly, the elemental eight are a little bit resilient to this, because they have elemental powers in addition to their generic magical powers. And riders are inherently of the light element, so depending on the anti-magic used and the type of rider present, it may not work on them. (Ruby is particularly resilient, both because her preferred form relies on magic and because vampires are not classified as magical beings and she's half-vampire.)
However, generally, it is the most effective method of shutting a rider down. If the rider hasn't transformed, the anti-magic will ensure they can't transform. If the rider has transformed, then the anti-magic if it's strong enough will instantly remove their transformation. (Some passive abilities of spirit totems will remain. For instance, a rider won't become naked upon their power being nullified, even though their clothing is technically magical in nature. This is because spirit totems are sentient life in of themselves: anti-magic can never cause sentient life to shut down. And because the spirit totem offers many abilities, they remain partially functional even during the worst barrage of anti-magic.)
But the transformation itself? That remains something that, bluntly, is NOT to be screwed with.
I also wanted to cover rider morality. They cannot accidentally kill someone, yet alone intentionally. This is the driving rule of a rider. If they are physically capable of acting to save someone, they must, even if said someone is a villain. If the villain is about to suffer a gruesome fate, the rider must save them. A rider can also not cause a chain reaction which ends up killing the villain. (Though, a chain reaction grievously wounding them is possible.)
If the villain causes a chain reaction, the rider is still compelled to try and save the villain. Sure, this doesn't mean they necessarily will succeed: there's no guarantee the rider reacts on time, and even if they do, if the villain actively refuses help (be it out of pride or desire to harm the rider), not much the rider can do. (Sadistic choices are also something riders don't have much control over: they'll do what they can to save both, but will ultimately be choosing to focus on one, with saving the other being on a "do if you can" basis.) Though...even if a rider COULD, no rider actually would leave someone to die.
This is because riders are, inherently, people of strong morality. As in, whatever method their spirit totem manifests as (from within, passed on from another, randomly found), no matter what, the spirit totem is only drawn to people who have that strong sense of morality in the first place. (A spirit totem cannot, after all, change the natural moral inclination of a person. It does not make a person good; it only appears for people who already are good.)
This, incidentally, is why it is notoriously difficult to get a rider to do evil. Is it possible for a rider to do evil? Sure, through several means. Brainwashing for instance. For a rider to willingly do evil, though, it's much harder. For a start, riders have a natural sense even without their spirit totems (who, as a reminder, are also spirit advisors, and thus, instantly know the difference between good and evil) for what is good and what is bad, tying into the above.
For them to be deceived into doing evil is therefore fairly hard, and for them to want to do evil even harder. Basically one of the only ways it's possible is if the rider in question can be convinced that the act of evil can be for a greater good. Even then, though, that's shaky grounds, because there is one other anti-evil measure inherent to all riders: they all have a very strong personal code of conduct, and for them to do evil, they must not be acting in violation of that code.
And this means riders are extremely resistant to evil in general. It's not impossible, but it's incredibly improbable, which is one reason why the traitor arc is feasible in the first place: Ruby didn't suspect a corrupted rider because of how unlikely it would be. When she's given cause to believe a rider has been compromised, she devises a plan which if successful would let a rider's natural inclination override whatever was causing them to be corrupted in the first place.