Assault and property destruction are already against the law. Banning organizations is unconstitutional as per the First Amendment. Antifa isn’t an even an organization. It’s a set of principles, beliefs and tactics around which a loosely allied decentralized network organizes.

If you are advocating designating antifa or any other groups as a domestic terrorist organization in the same way ISIS is designated an FTO by the state department, then you favor a position diametrically opposed to a fundamental liberty, therefore you are not by definition a libertarian but an authoritarian who merely wants lax regulation on the private sector.