Delete League
Players
Players can quit a league by selecting the hamburger menu icon and selecting 'Quite This League'. The player is then prompted if they want to be completely removed with no indication they were in the league, or they can choose to quit the league but still show up in prior stats such as Standings pages.
Managers
Managers can delete or remove a league they created by selecting Manager -> Full Control Panel, then under League Control select Decommission League. The manager has the option to simply deactivate the league but still be able to view it, or totally remove the league from view (decommission league).