Found a burn mark on the seat of our jobsite porta potty. Smokers are lighting up in there and I need to shut it down

Walked the site this morning and there's a scorched ring on the seat and the whole inside smells like an ashtray. We have one standard unit for a crew of about 14, and a couple of guys have been sneaking a cigarette in there when it's cold out because it blocks the wind. I don't want to find out what happens when a butt lands in the waste tank vapors.

I already told the crew to knock it off, but I'd rather have something firmer than a lecture. Is there a sign or a rule you can put on the unit itself? And if the seat is damaged, does that come out of my deposit or is it just wear? Anybody else dealt with this on a site?