That's crap. I'm against ever vetoing trades EXCEPT for when collusion is involved. That stuff kills fantasy football.
If the collusion involves trades with the plan of a trade back, the trade back should be blocked by the league manager.

If it's other wackiness, I'd finish the season, but not play with them again.



Tangential rant:
Playing with non-competitive people is draining for those of us who like to compete.
A 10k payout or a 0 cent payout is the same to me. I want to win and I love it when everyone else in a league wants to win too. It's very frustrating to see multiple startable players as FAs while other league members inexplicably carry 2 kickers.
Yeah, my team will be better than those teams, but it's the same as beating a kindergartner at chess. There's not as much fun in it.
Play to win or don't play at all.