You might also be looking for the TNG novel "Red Sector".

In the terminology of diplomacy, a "red sector" is an area, of varying size, that off-limits to a government or group of governments as a result of a mutual agreement. The Federation has strict rules in place regarding contact with a red sector in accordance with the Prime Directive.

In the 2350s, the government of PojjanPiraKot expelled all offworld representatives from their planet, leading the Federation to declare their world as existing in a red sector. (TNG - Double Helix novel: Red Sector)