[Alneyan]

You will not break communication with other Empires as long as it possible to reach one of your colonies, no matter how complicated this way would be. And you don't necessarily need to have explored the systems on this path.

I admit I don't know what happens if you completely block all your systems save one, but I gather it would still work as long as you retain control of this system. But of course, it would fall under "abusive turtling" as your prospective enemies cannot reach most of your colonies.