When they show up at a "protest" rally with weapons, helmets, masks shields and body armor they should immediately be told to leave. If they don't leave peacefully they should be arrested.
As a side note, whoever the judge was that agreed at the last minute to move the permitted protest location was an idiot who should be thrown off the bench. Also the lies of the Virginia governor that his State Police were "out equipped" by the protesters and so they let a lot of the violence proceed unimpeded while they watched should not be repeated. The lessons states and localities should draw from the Charlottesville fiasco is that people who engage in violence need to be arrested immediately.