In ISPConfig a IP address is either visible to all clients (if no client is assigned to the IP) or one client.
What I want to avoid is someone being able to select the asterisk when they're adding domains.
The asterisk is a wildcard and not a IP, so its not affected by IP settings. The asterisk is always visible to all clients. Normally you would use the asterisk for all websites and assign only IP addresses to specific clients that need a sl website.
A function to disable the * wildcard will be availble in ISPConfig 3.0.5 as you can see in the roadmap: