First stop exim in your WHM under services, just unclick any exim services running and save it.
Then open you ssh client and use this code:
/usr/sbin/exiqgrep -i | xargs /usr/sbin/exim -Mrm
Then restart exim.
This is much better then the mail queue system cpanel has installed, about 4 years (2009) they changed it and it’s just horrible now.
But thanks to the nice little code you can do it much faster from ssh.