Wait, wat? You want the man to use the force of state on some local anarchists? The mind boggles.
These aren't anarchists.
They're domestic terrorists.
Anarchists do not violate the Non-Aggression Principle. Don't be the first to initiate aggression, and everything is A-OK. The respondent/defender is then always justified.
Someone with a hammer in a crowd of protesters has not yet violated the NAP.
Someone with a torch in a crowd of protesters has not yet violated the NAP.
Someone using that hammer or torch to destroy a structure, and that action has been unprovoked by the property owner, has violated the NAP. Apply force. Increase force until the NAP violation is resolved.