You can alter the template in the admin cp and delete the options for IM ids in a members profile or you can have your settings so that profiles are not available for viewing and the IM id options dont appear at the top of members posts.
Good idea. I never thought of that.
Along these same lines I've been reading how others have fought off such problems and tripped across these this website at http://www.neilgunton.com/doc/spambot_trap and he has some good info/ideas. Especially his references at the end of the article.