There is probably nothing you can do about it now, but in the future make sure your league constitution has a rule that states they must start their best players. In my league, it goes as follows:
VI. Each team in the PBFFL is required to set their starting lineup every week with what they perceive as the best available players on their roster.
This avoids a purposeful tanking, and if a team were to do that, they would face disciplinary action in my league. I will admit it's a grey area and totally subjective who the best players on a roster are. But, in the case of obvious tanking like benching known commodities like Adrian Peterson and Ray Rice for bench fodder, it's an easy call to make. So, without knowing what rules you might already have in place, it's tough to recommend a course of action. There is also an alternative viewpoint that says a team owner should be able to set their lineup however they want, even if that means purposefully taking a dive. After all, real NFL teams often sit their starters during the last week of the season if their playoff spots are clinched in order to give them extra rest. Isn't that sort of the same thing? I mean sure, NFL teams are purposefully taking a dive just to make sure one team doesn't make the playoffs, but they do sometimes put themselves at greater risk of losing. And after all, fantasy football typically tries to mimic the real NFL in many aspects, so why not this one?
There's no clear answer here, but I think you have to go with whatever league rule is already in place and live it. If you don't like the rules, then be sure to change them before next season.