How to clear out /var/spool/clientmqueue

I am using CyberPanel V 2.1 on CentOS 7. I recently ran out of disk space on my system. When I investigated this, I found that /var/spool/clientmqueue was occupying 4GB of disk space (as revealed by issuing “du -h” in /var/spool).

What to do to clean out that clientmqueue directory.
Any suggestions? Please!!