To delete a user from VPOP3, go to Users → Accounts, select the user you wish to delete, and press the Delete button.

You cannot delete the user you are currently logged in as. If you do wish to delete that user, you need to set one of the other users up as an administrator, then log out of the VPOP3 settings, and log back in as the other user. Then you will be able to delete the previous administrator.