Golden rule of vetoing: Only veto if you suspect collusion.
Deal one is very suspicious, but you can't really tell without looking at rosters. Even then, you really need something more concrete to call out collusion. For instance, what does the team giving up Mauer get out of tanking? Is this a money or keeper league? If it's a redraft, you have to think this is just a bad deal, and you can't legislate the perception of value. If you try to do that, the whole system will fall apart (as you've witnessed).
Deal two is not even close to veto worthy.
In the end, what it often comes down to is that owners see a bad deal and blame the parties involved instead of blaming themselves for not making a competitive offer that would have swung that player their way.
Here's a suggestion: If you think these deals are really bad, require a 48-hour trade posting period. This isn't a veto period. Instead, any other team is allowed to make a counteroffer. If someone offers Pujols for Mauer and the owner still takes that crap, then you know you have collusion, and the veto is assured.
Give that a go. Let us know how it works out. Good luck!