When you're in the rental business, this is what you have to put up with.
Why don't you have a talk with your tenants, expressing your concern about the recurring condition. Maybe you need to teach them how to use a GD, what can or can't go in. I've had tenants who thought they could sharpen their knives in the GD. Worst case scenario, evict them on the ground of being terrorists. Your plumber will be witness #1.