I think this situation is collusion, but like others have said I don't think you can do anything about it.
For me collusion comes down to two things: Did multiple owners work together and subjectively, did one owner clearly not benefit from the working together. For instance, in trade situations, owners are always working together, but they usually agree on a deal that both feel benefits their team. Even if a trade looks lopsided, if an owner can explain his rationale for making the deal, and it makes sense, then it should not be voided. If however, an owner does something that they can't explain how they feel it would benefit their team, and they were working in conjunction with another team, then collusion is a lot more likely conclusion. In this case, if I were commissioner and someone complained about collusion, I would have each owner who picked up a RB explain their rationale for their pick-up and how much they paid. "I wanted to screw the top team in the league" would only be a valid reason for an owner if in some way that benefited his team (i.e. if he was playing the top team in the league that week, and it improved his chances of winning).