I believe the question is about limiting access to a Tor bridge (relay?), not an Onion service. With that said, firstly, I don’t know a direct solution, but:
- I know that in order to use a Tor bridge you have to specify its fingerprint and certificate (see https://bridges.torproject.org/bridges/). But I don’t know whether this makes a bridge unusable if you don’t know these values beforehand. If it’s the case, then nobody can use your bridge by default, as long as
BridgeDistributionisnone. - I’m curious to know what your use case is, because it seems unrealistic to me that someone would want to scan random IPs to find a bridge just to use it, because why would you do that when you can just use one of the thousands of public Tor relays.