the network services page allows you to open a service port to the public for instance but it’s not user specific. I am afraid there is no general frontend user specific access page, but there is a delegation module for the admin interface.
It depends on the service if you are able to manage user specific access. In Webtop for instance you may disable a user so he can’t login to Webtop anymore but he can use all other Nethserver services.
In OpenVPN I found no possibility to disable a user without delete the VPN account.