The new rules are simple and clearly defined - Invading another user's privacy is distinctly prohibited nowadays. However, once upon a time it wasn't, and that certainly made for better beefs, IMO.
User beefs can be handled in a variety of ways : The first way would be to simply place the user you have on your ignore list and be done with it. The second way would be to take it to the dome where you can air your grievances in public with the added benefit of providing some entertainment to your fellow users. The third and final way to resolve beef would be to appeal to an admin who would then give both users a stern warning to essentially STFU in regards to one another or risk account termination.
Those are your official, staff-certified means of putting an end to this. If you choose to ignore those options and go the invasion of privacy route, action will most likely be taken against your account.