Darn it, flykiller, I've been telling myself to stop raining on other people's parades so much, and then you post something like this. 
I'm sorry to do this to you, but...
"So we'd give some extreme navigational hazard as the reason for the interdict. Like Nirton, only even more so. Trouble is, the system must have been visited by dozens of explorers over the last four centuries, starting with our own Scouts in the 1st Century. Reports must be filed in scores of library data collections and private files."
"No, an interdict is the only possibility. But we'd have to spend the resources to give it a lot more bite than our average interdicts. No merely visiting patrols. A couple of squadrons of escorts permanently stationed, I say."
Hans

I'm sorry to do this to you, but...
"what, redzone the star system? that won't work, we'll have every harebrained adventurer and desperate trader jumping in-system just to see what we're hiding."
"So we'd give some extreme navigational hazard as the reason for the interdict. Like Nirton, only even more so. Trouble is, the system must have been visited by dozens of explorers over the last four centuries, starting with our own Scouts in the 1st Century. Reports must be filed in scores of library data collections and private files."
"Every professional and amateur astronomer on worlds for many parsecs around might decide to take a direct peek. Sooner or later someone would, and probably sooner rather than later. Besides, the system is already in all local astronomical databases. If we don't include it in the Survey, we'll shine a spotlight on it right away.""we take it off the list before we submit the master survey." bright, happy smile.
"that's ... that's crazy! you can't hide a star system, it's right there for everyone to see!"
the senior admiral stirred. "after the master survey charts are submitted, does anyone look at anything but the navigation data?"
"No, an interdict is the only possibility. But we'd have to spend the resources to give it a lot more bite than our average interdicts. No merely visiting patrols. A couple of squadrons of escorts permanently stationed, I say."
Hans