In my city, face masks are required by emergency order in all public places with violations subject to a $300 fine. Though the vast majority of people have complied with this order (indeed they mostly complied before the order went into effect), a stubborn few refuse to wear masks.
Generally calling the police about this is not effective, because the violator is long gone by the time they arrive.
What can I as a citizen do about this? Can I detain the violator? Can I sue them for endangering my health and the health of others?