All you can do is give them a strict warning, and if it persists, ban them completely. I think this is Hollowqueen, who did this on RotR. I gave them a strict warning, got some BS answer that was denying it. After some new members come on, ask them if they get the same message. I got the same one, Cable, as did Zenke, Parius, and a few others. If those new members did get the message, ban the person. I told them off and I got the same BS answer as with Hollowqueen. I did that with them. Any advertising via pm should be reported to the staff, which I did report it to Keten. I'm just saying what I did to Hollowqueen, action with this person is to be determined by staff. But if it doesn't stop, you have to ban them. It just isn't right.
Sorry for posting here if I'm not suppose to, just mentioning how I dealt with the exact same situation.