This calls for a complex answer, Elena......excellent question!. There is so very much to learn/know about Hypnotism and luckily much has been written on it. The true "Experts" could best answer this and BTW, as much as I know, I'm far from an expert. But I will gladly give your question my best shot.
In terms of one on one (or very small group) you would have to know before hand that a person intends to hypnotize you, in order for you to be able to put up a defense of any kind. In other words, really good hypnotists can put you into an hypnotic state w/o you being forewarned or aware. It's not easy, but is done all the time, especially if one knows next to nothing about the topic of hypnotism. In these cases, you can have been hypnotized and not even know it happened....and the suggestive direction can still have taken hold. Ethical hypnotists don't even consider doing this. It can be done sometimes for entertainment purposes, before an audience.
Presumably, if you have sought out a hypnotist by choice for a specific reason, there would hardly be reason to want to stop them. Although, for example, if you had much apprehension and were highly anxious about doing this, you could possibly choose to ask the hypnotist to STOP, due to changing your mind, etc. It would depend then how far into the procedure you were and would also require the hypnotist to make sure you regain your original consciousness completely. As I'm sure you've heard, there are individuals who simply cannot be hypnotized. There are numerous reasons for this. On the other hand, there are those who are HIGHLY susceptible.
Another tidbit of information, useful to know; there have been many studies done to strongly suggest that the brainwashing methods of people such as Cult Leaders, is in fact, a blatant form of hypnotism. Performed daily, it is easy to understand how the average person can be nearly overtaken.
I should stop now....before I hypnotize myself!! But basically, it IS possible to stop someone from hypnotizing you, if you're armed with some knowledge. Seriously, no one can take full charge of your mind but you..........