Should post something, having been involved a few times. Once I got emailed some questions by an individual, asked for help. I was suspicious and was able to find the longer English phrases from a question on the web. I wrote to the university, eventually the (graduate) student was identified. It was just a slap on the wrist, though, there is enough embarrassment about such a thing that a mathematics department is likely to do little that would allow publicity.
More recently, questions from my publications showed up on MO. I could think of only about three people who would have any interest in teaching my material. I guessed incorrectly, but it was one of them. Alex was teaching a small seminar and knew immediately who had posted the questions. Also, it was a sort of take-home exam, more precisely he told them that the final would be selected from that long list of questions. He told me that the offending student showed up looking green around the gills (after I had made some warnings on MO). All Alex did was make a mild announcement at the end of class that the questions should not be posted online.
More intricate was a girl, maybe three years ago, posting contest questions on MSE under a half dozen usernames. I was only peripherally involved... Eventually it was figured out, and her invitation to the USA summer workshop (for Olympiad hopefuls, I think) was cancelled. Fellow came to tell me about it at the 2013 Joint Mathematics Meetings in San Diego, California.
The worst one for me is still unresolved. Some relatively senior graduate student kept posting disguised versions of a major conjecture in matrix theory, deleting them, changing his username, posting again, both on MO and here. We think he only stopped because he finished his Ph. D.. If he went into industry, or a postdoc with a group that did something specific, he would have no time for that problem.
Oh: in all cases, MSE or MO gave me no information on the miscreants. Took action on my own.