What gets me most with many alter egos isn't just their delibrately senseless and stupid posting. What's worse is that they make the good posters lose interest and recede from posting. If that isn't stopped this site won't be a valuable Bowie board for very much longer.
As the technical means of identification are not in the hands of the moderators I can only try to stop the alter ego influence by not reacting to their posts. I have tried to do that for quite a while now and have pointed out to others that they are answering to an alter ego, hoping they would stop to answer as well. Of course there are too many alter egos around to avoid them completely, but as soon as you get the impression that you are dealing with one of those things I would recommend to just not reply. In the long run some of them hopefully will then get fed up with threads in which they only interact with themselves.
Thanks for the good advice. I'm glad the mods acknowledge that this is annoying.