AirPort Base-station Visibility
Someone can downvote, edit or comment if I'm wrong on this, but I don't believe this is possible.

Normally, you can only see the AirPort from inside the private network, and by default it doesn't advertise or allow configuration on the outside interface (i.e. the internet). Because you're using bridge mode, though, there's no distinction between the networks. To prevent it from advertising its existence, you'd have to disable Bonjour entirely on it. The utility doesn't have an option for this, as far as I can see.