This also happened in my main league 2 years ago. My cousin (and roommate at the time) was actually the one who intentionally tanked to "get a better playoff matchup." Because he scored a lot of points during the year but lost some high-scoring games, he could have finished anywhere from 3rd to 6th based on just the results of the last game, and he liked the matchup as the #6 vs. the #3 seed best. Being the commish, and without any rules against it, all I could do was chastise him for his lack of sportsmanship and deal with the awkwardness of watching the Week 13 matchup play out as one of my close friends in the league fought to gain the same final playoff spot that my cousin shouldn't have been contesting anyway.
In his case, it normally wouldn't have been absolutely clear-cut, but on Sunday he did pull Reggie Bush, who ended up having 3 TD's, for Nick Goings in Monday night's game. He basically told me before the game that he was trying to tank to choose his playoff spot - as a silent protest regarding being the high-point scorer and still being 3 games out of first. At the last minute, due to my pressure on him to not be a total d00shbag, he replaced Goings with DeAngelo Williams, but still lost - would have won with Bush, otherwise.
Luckily, it ended up not mattering a WHOLE lot...the friend who was trying to seal the 6th and final seed ended up not winning anyway, avoiding the issue of my cousin's intentional tanking costing him a spot.
My cousin ended up moving away before the next year, so we avoided the issue of hard feelings at the next draft...though some bitter comments still came up at this last year's draft when reminiscing. We ended up adding a "sportsmanship/honest effort/fair play" clause to the rules the next year to give some clarity to the situation.
Plus, my cousin ended up getting THRASHED in the first round of the playoffs, where he would have won easily in the first round had he finished his projected 3rd. Going back and looking at the potential matchups, had he just let things play out naturally, he would have coasted to a championship. Poetic justice, I suppose.