In America we shouldn't do this. In Germany which has a history of Nazism it is completely appropriate to ban neo-nazi or fascist political parties.
Context matters. Ideally there shouldn't be bans even on extremist political parties but the circumstances of an individual country have to be taken into consideration.
Banning a political party is only okay if their actions are directly harming a person or group of people. Being extreme while causing no harm is not grounds for disbandment. Promoting violence against a race that leads impressionable party members to commit crimes is probably when the government should step in and end the organization. It is not exempt from responsibilities other organizations see just because it is in the political realm.
You may not agree with extremist political parties, but that doesn't mean it has to be restricted. I don't believe in a lot of what the Republican party stands for, but that doesn't mean I think it should be restricted. Sometimes they seem a little extreme. However, it is a part of the democratic process. Everyone should have their voice heard. It is for the people to decide.