previously we have talk about the ways to remove whm mail queue through SSH but in case of huge mail queue it will take hours to clean the queue. In that case the most protected thing (and the most unpleasant one) you can do is remove all files from EXIM input folder that uses Exim to keep the mail queue .
Run following commands one by one:
/var/spool/exim/input rm -rf foldername
This will help you to clear huge mail queue within less timeline.