He hasn't been convicted of a crime.
What about prevention of a crime?
you want to give the state the power to remove people's individual rights, to arrest, detain, stigmatise people in order to prevent a crime that they may or may not commit in the future? that is a dangerous precedent.
I am sure Mr long-legged mack daddy is would love to use that power to arrest and imprison every potential terrorist (i.e. conservative, patriotic American) and to seize every firearm "for the children."
It is easy to be in favour of "free speech" for nice people, it is the hard cases such as the rights of Nazis and paedos that is the true test of your commitment to free speech. This man may commit a crime in the future, at that point he should be killed. As he has not ben convicted of anything, the state cannot touch him. If I give the government the power to infringe his rights, I am implicitly accepting that the govt has the authority to infringe mine.
That said, I agreed with your overall sentiment, I certainly wouldn't trust him and I would not let him anywhere near my kids.