If there is anything in your league rules about collusion, I think you can jump on this. If collusion is only mentioned regarding trades, you might be stuck, but I agree with Snakes -- they made a phantom 'trade'. I think that could be the 'in'.
This would also be a convenient way around a trade deadline, which you would not allow, so why is this different?
I'd offer for them to reverse the moves or get the boot.
Waivers would have helped, but that does not make it less cheating, IMO.
It is also tough that you are the one being colluded against. You might have to do a league vote or something 'Did these teams break the collusion rule?' and 'Should commish take action?' and see what happens.
Be excellent to each other.