“Dealer/service Plate Permanently Affixed With Screws?”

Years ago you were not allowed to use screws/bolts to "permanently affix" a dealer plate (when they were yellow). Can anyone clarify if it is in fact legal to use screws? It's off topic, but I take my plate off at night so it doesn't get stolen and the by-laws are giving headaches. They 1, refused to take down my info, 2, said they are going to take my car and 3, said municipal law supersedes provincial laws. I had a rather bad day and these guys are coming here twice a week to bother me. Can I mount a dealer plate with tamper proof screws?

This was frowned upon, we used to use magnets or bags and then plates went missing all the time.