This question actually covers two different scenarios. First, there may be some bowlers listed as Temporary Substitutes that have not bowled this season and you simply want them removed from the Sub List. Second, there may be bowlers who have permanently left the league and are not expected to not return.
BLS provides options to address both scenarios in the Bowler Maintenance:
1) Not a League Member: Select this option when you if you want to keep bowler information in the league file, but not have the bowler listed on the Temporary Substitute List.
2) Quit the League: Select this option when you want to permanently delete a bowler record from the league.
Note: Bowlers tagged as "Quit the League" will not be removed until opening Utilities > Delete "Quitters" and unused VACANTs. However, if there are any scores or financials associated with the bowlers that have Quit, the bowlers will not be completely removed until restarting the league for next season: Tasks > Restart League for Next Season.