The same way any other federated system does. Allow, ignore, block and such... The whole point is you get to choose. If someone happens to get a lot of requests from other peers they might be considered a reliable source.

However you would probably want to have them return ranking information allowing for re-ranking on the caller including its own system and others. Perhaps returning this could be optional?

Or even have the federated searches share portions of the index when peering others. This would allow one system to determine if the ranks appear in line with expectation. Allowing one to trust but verify.