A bit off-topic, but in most cities there is an ordinance against burning something (leaves, or whatever) without a permit. Why not just cite the protesters for that?
I’m sure these ordinances provide for stiffer penalties if the original citation is ignored.
Of course “sanctuary cities” would not enforce that ordinance, but they would also ignore any other law the flag-burners might break.
I've been asking what you suggest for awhile, no answers: Perhaps liberal maniacs really think it's okay to burn the flag. Is it okay to violate fire laws?