Well, if this is someone with a grudge, it must be an incredibly tiny one--you just have to pick up a cup once a week, right? I can see how you'd want to do something about it on principle, but anything you do to try to catch someone is going to take way more time and effort than just continuing to deal with it. (Not to mention how stupid you'd feel if you made a big thing and it turned out to just be random strangers.)