I would tell you absolutely not. Do not limit it. Every Middle Eastern regime is testament to the fact that when they limit free speech, they push them underground. They will still preach what they want and they won't have the antiseptic of sunlight. Just as I don't want my speech prohibited by this false banner of Islamophobia or anyone's criticism of Islam, similarly the light of day can only be shed on the radicals and we can monitor them, if they have the freedom.
As long as they're not preaching imminent violence and violence against individuals, our Supreme Court has deemed that theirs is free speech, even for the KKK, and I think most societies have shown that when you limit that speech, you actually radicalize them more.