I think the point of many here is that there are two potential issues. That they may be messing people up and their methods. The correct approach to deal with it depends upon which issue, or both, you are trying to deal with. If you are dealing with people getting messed up then you are almost certainly best off using letters to the editor over placing ads. The first thing someone who sees an ad will ask is "what is the motive of the one who placed the ad". With a letter you do not have this question. The most obvious answer that people will give to the motive is that he is afraid the loophole will get closed. In that case the one who placed the ad may be worse than the one doing it. He is then like the ganiv who snitches on a different one who is getting the better loot since there is none left for him.
If the issue is his methods in and of themselves then for someone who uses the same methods to place an ad is certainly being hypocritical. It would mean that your issue is either again that he is messing it up for the others who use the same on a smaller scale or just that you feel you can do it as long as you don't get caught.
There are some here who do not wish to be so hypocritical and are only willing to participate if there is a clear issue of people being messed up in ways that they did not understand and would not have taken that risk. If the person understood and was ready to take the risk then there is no problem.
If the issue is that he is signing up for someone else with permission then you are getting into an area which may be illegal but not unethical and may also not warrant ads. They also will not be too effective when the issue is something that people feel is not wrong.