They have the same rights as you. Just because you don't agree with them and they're idiots doesn't mean they lose freedoms. If you do take away anybody's rights, it will turn into a slippery slope that should not exist. Take out the politics and hateful as they are, White Supremacists still have free speech.
While we should defend the rights of people to free speech, there should also be exceptions to this rule. Infringing on the rights of others, as white supremacists do with their hate speech, is not something that our Bill of Rights endorses or guarantees. Freedom of speech should only be allowed when it is not somehow restricting the rights of other people.
I do not believe that the ACLU should defend the free speech rights of white supremacists, because much of what they say could be labeled hate speech, which is illegal. The ACLU should be defending free speech that is protected by the first amendment, not speech that may be illegal.