Wow. I must admit that I'm guilty of being a screen-toucher. It's not out of ignorance or disrespect or anything like that; I've been around computers my whole life. I have no idea why I do it, but I've even done it a couple of seconds after someone asked me not to. I don't get annoyed at all at people when they tell me not to, I get embarrassed and sometimes annoyed at myself for having done it (especially if it's repeated).
You should just not have conversations near other people's monitors about what's on their monitors. Ask them to come to your cube, or take a screenshot and circle the spot in question.
