Queue management for postfix is done with the postsuper command, which you can use to delete mail in vareious scenario’s.
#1 Remove specific mail
Run the mailq command to list pending mail, en get the id, next you can delete a specific mail with:
#postsuper -d [queueid]
#2 Remove ALL mail
#postsuper -d ALL
#3 Remove all deferred (stalled) mail, without touching mail that has had no delivery attempts yet
#postsuper -d ALL deferred