Setup of WHM/Cpanel security and backups

I have a VPS server with WHM/ Cpanel running on CentOs

One of the account has been sending out 100's of emails and I seem to have stopped this but now have a 56 page mail que that won't delete.

I need these removing and better security setting up on WHM and the individual cpanel account. Also any other security vulnerabilities you are aware of that can be fixed or easy enhancements to WHM.

You should be well experienced in WHM and SSH.

Thanks

Barry Reynolds