Once the scoring for a season has started, we do not allow users to leave leagues. We also do not remove users from past leagues, either.
Not only would it create issues for your league mates, but it can impact your league history and standings.
It's possible that you may see past leagues from previous years in your mobile app. While you can't be removed from them or delete them, you can archive them so that they're not visible.
How to Archive a League
Go to your list of leagues and press on this icon:
Then, you can press on the box icon to the left of a specific league to archive it. It won't delete the league, but it will hide it from the active screen so it's not visible.
Any archived leagues will appear underneath the list of your active leagues. You can make them visible again at any time by pressing on the plus icon.