I think it's ok to touch laptop screens. I touch my own and if I'm interested enough in pointing something out I may touch yours because at that point I'm at a completely different place mentally and not thinking about your little quirks. One of my co workers stressed out every time I touched his screen but it didn't make me stop. I'd avoid it when I remembered but I frankly don't believe it's a big issue. He'd let a loud annoyed sigh and reach for the cleaner that he kept by his desk and I'd laugh at him.
Your pet peeve is not really my problem.
Your expensive screen is not brittle and will not crack at the slightest touch. It will not melt under the powerful corrosive force of my finger oils. It just makes a little streak. Get over it and carry one of those fuzzy lcd wipey things with you if it bothers you that much.
Anyway, people like me exist. We will annoy you but it's (mostly) unintentional. You can try to point out your annoyance in all sorts of amusing/passive aggressive ways but it's not going to completely work. It's still going to happen. Realize that you're being a bit irrational and get over it.