let's examine the long chain of assumptions you're making:

• the first assumption is that "white supremacists" want genocide. a fringe element of white supremacists want actual genocide. the rest are on the spectrum somewhere between being racist against others and being white-positive. but let's assume "all white supremacists want genocide". let's assume you're right in the best case scenario for you moving forwards.
• the second assumption would be that a group of people advocating genocide is any worse than those people simply existing. in order for this to be true, it would require for otherwise reasonable people to listen to a lunatic calling for genocide and think "oh wow he's right. i'd better do that". like, if the message isn't getting through, then they are not a threat any more than they are just by being alive. but let's assume, again, that a normal person listening to a small group of retards is going to change everything and decide genocide is good.
• the third assumption is that this will lead to violence. even if people want genocide doesn't mean they'll cause genocide. i want a million dollars. that doesn't mean i'm going to get a million dollars. but again, we'll assume that this is the case and that shouting for genocide increases the chance of it happening in the future.
• the fourth assumption is that using violence to suppress these people will actually have an effect on them. why would people who believe in genocide be stopped by opposition? what are you accomplishing by oppressing their freedom of speech and assembly? all you're doing is giving those people a "them" to hate. people who go to a rally like that will be radicalized more when faced with concrete enemies rather than abstract ones. you're giving them a specific hatred.

i can go on.

but really, you're completely wrong about all of this. if your goal is to stop whatever those people want from spreading or happening, the worse way to reach that goal is by energizing them and violating their rights in the process. the best way to deal with these people is to simply mock them and ignore them. and no, you can't say "but that's how the nazis came to power". no, no it is not.