Wow, that one sounds really persistent. I would probably use NRaas Register to remove their assignment from that role, but of course the game will likely just assign or spawn a different NPC sim to take their place. I am certain there must be mods available from other sources that suppress the reassignment altogether.
It sounds like they really like your household, though. One of the fun things to do with an overly friendly mascot is to roll with it and have one your sims seduce them with the goal of seeing whether they keep their mask on during woohoo and then later if/when they fall asleep. I won't give the answer away. :)