If he left his QB spot empty, then it our league it would be in violation of one the league rules in our constitution that states you must field a starting lineup of X, Y, Z. If he swapped out Brees for a lesser QB, then it's a legal move. It all depends on the rules you have setup in your league. If no official rules were broken, then it's not cheating necessarily. It's worth pointing out that we use ESPN, and it's literally impossible to leave a spot vacant, except in the case where you drop someone from your starting lineup.
If a guy is in position to purposefully lose AND still make the playoffs, then that's his prerogative to do so. My league is pretty competitive, so every win is vital for pretty much every team. Taking a dive for a better seed wouldn't really factor into it. Collusion is so hard to prove, that if you're going to discipline someone for it, then it has to be an obvious rules violation. One of the most important things you can do is have an iron-clad constitution. Here's a link to ours: http://www.mediafire.com/?tp5x8mvf3hsxvn2