something more to add - it seems like the aliases will be deleted after one day automatically :
DROP EVENT IF EXISTS clean_spamalias;
CREATE EVENT clean_spamalias
ON SCHEDULE EVERY 1 DAY DO
DELETE FROM spamalias WHERE validity < UNIX_TIMESTAMP();
So if you can’t wait and if it is urgent then just renew the timestamp or delete it yourself as already mentioned or just wait since it should be deleted automatically 24 hours after the expiration timestamp is reached.