What I have done is prevent the ability of a peer to create a room depending a hash value in the URL. For example, if my game URL is "awesomegame.com", to be a host the peer would need to enter a URL with a "secret" hash similar to "awesomegame.com/#secretHash" or "aweseomgame.com/#host"
Here is how to use it:
After logging in, but before joining a room, add a System condition to compare two values. Compare Browser.Hash to whatever you want the "secret" to be. Compare: Browser.Hash Equal to #mysecret
If the user is allowed to be host, create a new room with a unique name. For testing I simply used the Multiplayer.MyID for the room name.
If the user is not allowed to be host, get a list of rooms. If no rooms are joinable, tell the user that there are no Hosts or servers, or whatever message you want to say... or tell them to send you an angry email