I think veto systems are important, but there are definitely a percentage of players who will veto any deal that they feel improves their competition. I probably only veto about one in ten trades that I see, and usually only do so if I see that one player is giving up a 1st or 2nd rounder and getting average guys in return.
If I see a trade where a guy gives his 5th rounder for another guy's 3rd rounder I'll leave it alone, but if I feel like one team is forfeiting essentially no value while acquiring a first round talent, that's where the veto comes in.
You should post some of the trades that were vetoed so we can give you input on whether some of them were reasonable, or if your leaguemates are just totally out of line.
If you're saying it's basically every trade, then it probably is the latter