I find all it takes is one member to post a message on the board explaining what the other manager is trying to do and educating all the other players to nip this sort of thing in the bud. It happened in my league a few years back and now this guy has to work pretty hard to get a trade done. Whether it was fair or not for us to do it to him, we decided to call him out and alter the perception of him and now people are very skeptical of his trades. Muhahaha!
But yeah, you have to let people do what they want but you can take a trade proposal from them and publisize it, ripping it apart if you like and ripping them apart at the same time - Thus educating the other managers. Don't use your role as a commissioner to handle this though - just handle it as another manager in a mode of smack-talking... That's what I'd do anyways...

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