Even just sorting out from who's "tanking" and who is just a garbage manager would make only a pursuit, never an attainment.
It begs the question, what about when the inevitable happens and you penalize a bad manager that really isn't tanking. It could make a bad situation worse and he quits the game. Alienating and penalizing active users more often then not is lose/lose.
"Tanking" is a strategy. Regardless of which strategy is applied, the manager is still actively managing. That is where it begins and ends for me. I see plenty of shoddy managers that drive down league quality without tanking. There are even complete leagues that are like that.
Ok... What about some form of system/ranking thing for managers.
You receive values/scores based on managing ability. Decisions that are good, winning games you shouldn't have, losing games you should have won. Mismanaging a key players minutes and messing up their game shape. Expanding luxury boxes when you're not even selling them out already, etc. Things like that.
Higher rating better, worse rating/negative rating worse obviously.
with this, can determine good managers from bad managers. Then can determine tanking. Have a thing where you can see their "managerial" history, just like scouting a team's schedule or roster moves etc. If they had a rating of 800, but 2 seasons later are 330, yeah... obviously they've been tanking. If they are at a 30 and don't do well. Maybe they just plan suck.