So. As him >>19395 said BOs can exclude the board from the boards list and the top boards index.
This does not exclude the board however from the overboard, the latest posts, and the new images feed.
Imageboards did not mean to be private chatrooms. Surely other solutions, platforms exist that suitable for that purpose. We're gladly host your content (as you said it is not illegal) but there are limitations, both of the engine and the format (the "genre" if you will).