Bumping this suggestion for fixing leaving and rejoining the league to gain an advantage in the upcoming season:
https://igpmanager.com/forum-thread/46152
This game is already time-consuming enough for league hosts. This issue warrants an official response, remedy, and/or guidance from devs.
One way to fix it would be not to allow managers to rejoin the same league for 1 week. I'm sure there are other approaches.
But when they click to leave there is a bold warning which informs the manager that they cannot rejoin the same league until 7 days have passed.