Unless you've got a bunch of undefeated teams, and teams without any wins and a season shorter than 14 weeks, nobody is mathematically out yet. But the best way to avoid this sort of thing is to set a trade deadline. Most of the playoff teams will begin to separate themselves from the pack over the next 2-3 weeks, but until then, it's still anyone's game. ESPN, for example, sets their standard trade deadline for Nov. 21st this year (Wednesday night before Week 12). If that is too late for your tastes, then set it earlier.
If you don't have a trade deadline, then you pretty much have to allow any trades, as long as there's no collusion or cheating (or sabotage). Even if a team is mathematically out of the playoffs, I know that I play for pride and historical stats. I've been in one of my leagues for the past 4 years, and I'm not going to just dump my team if I miss the playoffs. Obviously, not everyone thinks that way, but you shouldn't disable a trade simply because a team won't make the playoffs. Now, if the trade is obviously 1-sided and favors the playoff team heavily, then there might be collusion involved, in which case, yeah, you veto. But, generally speaking, as long as the trade is more or less even, and there doesn't appear to be collusion, cheating or sabotage, then you let it stand. Your job as commissioner is not to micro-manage the league, or impose your own valuation biases onto other team owners.
Simplest solution: use a trade deadline.