Groups whose primary purpose are to propagate immoral ideas are detrimental to society. As long as we're sure that disbanding them won't just make them harder to control, I say go ahead. This is not a case of a ruler suppressing dissent to stabilise his/her rule. This is a case of upholding the moral values we, as a society, hold dear. Hate groups should be eradicated.
I'm as sick of the morons over at the Tea Party as anybody, but they're within their rights in this country to be stupid. They'd be crossing the line if they actively started injuring people or something of that nature, but you can't make it illegal to simply have insane ideas in politics. The way we're trending you might need to start making it a requirement.
The other extremist groups that are much more open about their hatred (and thankfully have virtually zero politcal sway) are also within their rights as long as they follow the same standard of do no harm.
To ban a political party would not be in the good faith with what the constitution set out to accomplish. Everyone gets a voice, no matter how unpopular. If enough people are like minded, parties are formed. If I had it my way, we would honor George Washington's request of not forming political parties. The divide us in general. However, as a collective we seem to enjoy our current party system. Many things are unpopular before they gain acceptance over time.