Did you take their money when they started, or do they have to pay at the end?
If you took their money, I'd be a hard guy about it and say "I don't care if no one vetoed. You just gave your reasoning as 'if my team sucks, why can't I help someone else.' I'll tell you why. Because that gives one team more roster spots than the others, and access to players that others do not have access to." I'd consider just giving everyone their money back, or freezing the teams that are cheating and running them myself through the end of the year, just to make sure they're being competitive.
Those 2 players WILL ruin your league. If the team that's benefitting from those trades wins it all, it's absolutely unfair to the rest of the league.
Here's your trade off. You either piss off those 2 people, or at the end of the season, when the cheating team wins ANY MONEY AT ALL, you get to deal with the rest of the league being pissed. At the very least you can get in direct contact with everyone else in the league, and explain to them that you don't want to hear it at the end of the year, when the cheater takes the cash. If they aren't going to object, then they can just deal with an uneven playing field.