Are you trying to delete a domain owner account or a sub user (email etc user) ? If you are trying to delete a domain owner just delete the domain as that user is the control of the whole domain
If you check the “Edit Virtual Server” page, you’ll see that the “Deletion and disabling safeguard” option is enabled—this is used to protect the domain from accidental deletion—just toggle it off, and the “Disable and Delete” menu entry will show up again.