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 as whom you are currently logged into the VPOP3 settings (e.g. Postmaster). 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.
If you try to delete a user but get an error User xxxx cannot be deleted - in use, that means that somewhere, something is logged into that user's account, so the account cannot be deleted until it is logged out of everywhere.
In the VPOP3 settings go to Status → Active Sessions and look for sessions in that user's name, and then go to the relevant PC/software and disconnect it and/or configure it to use a different account.
A common reason for this problem is that the VPOP3 Status Monitor will normally be logged into a user's account and people often forget that.