To be able to delete OpenVPN only the last OpenVPN user account by following these steps:
On the VPN page, in the More column, select Configuration.
Select the OpenVPN tab. The Delete OpenVPN button will appear in the OpenVPN User list, only one user account remains.
Press Confirm to continue the OpenVPN removal process.
Wait until the process is complete.
A notification appears "OpenVPN configuration [vpn-name] has been successfully deleted".