If its a money league and you have already collected their entry dues then withhold their winnings. Also, you could shut down their teams or lock their rosters since you have commissioner tools. Let everyone know what is going on a vote on it.
my bad, thought this message was going out to people on my lower level. after further review i see other messages where commisioners are cheating. ouch! to try and answer your question then, unless $$$ is invloved why be involved with a league that someone would have two teams in. there are other leagues free leagues out there.
why complain, ask the question how could someone, and/or why would someone cheat to win a yahoo public league. how could one or more than one person(s) get together and get two teams in one league? unless it is after the fact, i suppose. i wonder how this mystery will turn out.