Go back into Preferences and look for the "Remove memorize payees" option setting.Is there a checkmark in the box to the left of this option setting? If not, click the box to put a checkmark in it.
Rebuild MPL
After creating abackup of your Quicken data file ...To rebuild the Memorized Payee List from existing transactions, click Tools,hold down Shift and CTRL, then click Memorized Payee List.
I was just playing with this and I might have an idea of what the problem is.It seems that if you lock a payee, it will not be deleted.Mine is set to delete older than 13 months:Now I unlocked a few, closed and reopened the Memorized Payee List.BTW you can sort by the Last Used column, and select multiple payees and delete them to clean them up manually.