Why not just stop player gaining skills for teams that do not promote.
If someone had a bad season, he should still have a solid team to start with for the next season and should be enough to help them promote.
Teams (not deliberately tanking) not promoting last season and winning the title the next one is highly unlikely and unrealistic.
So, even if you stop their players gaining skills, it should not be a problem.

To be honest, Im just writing this as a suggestion EVEN THOUGH i know Nordeus wont do a thing about this.
Pro Association tanking has been going on for atleast 10 seasons, if not more.
If they wanted to do something about it or cared at all about this issue, they would have already done that.
But why would they care?