I give them a friendly "heads-up" like: "Hey, just so you know, the owner will yell at you if he sees you (sitting/putting a drink) on the table. He's afraid it'll (unlevel/stain) it."
I've said this to all kinds of people, young, old, sober and drunk, and never had a negative reaction. Most thank me for the "warning". They might even remember the brief explanation about why "the owner" doesn't like it.
pj
chgo
This is probably the best way to handle a situation like this. Your next step is to let the staff deal with it or say nothing at all. Saying anything else makes you look confrontational and obnoxious which will ALWAYS have a negative response and you should never invite this into your life. It's just cloth.