If this has been suggested before, my bad. If not, well I'm suggesting it.
http://forums.spiralknights.com/en/node/6657 - The latter half of that thread is a good example of why such a button would be nice.
I also suggest putting it on the opposite side as Edit/Reply and making it noticeable in some way or other (such as having the button be colored red with Report being black).
Easy to use report buttons on forums and even in-game are a good way to encourage players to actually take the time to stop and report obnoxious and undesirable players. Removing these players from the game is a good way to keep your paying customers happy.