To remove a user account, go to the VPOP3 settings, and click Users on the top bar, then select the user(s) you wish to delete and press the Delete button.
You will be asked to confirm that you want to delete the selected user:
Press OK on this window to confirm the deletion.
If the user's mailbox is not empty, you will be prompted again to confirm that you want to delete their messages and delete the user:
Press OK on this window to delete all that user's messages, and delete the user.