The easy way is just have each instance use a different service URL (deriving it from some characteristic of the user session to make it unique - e.g. session logon ID).
There's no way to stop the WCF binding publishing its endpoint details (aka the real pipe name) in the Global namespace if it's running with sufficient privileges to do so. But maybe you could adjust the process token privileges to disable the SeCreateGlobalPrivilege before opening the service host, if your service finds it is running as admin with elevated privileges.