And it is absolutely still possible with NextBlock. A random stalker will still be able to see your posts, comment on your posts, and follow your profile to see your future posts in their feed.

On NextBlock, you just won't see any of their posts or comments.

If you want a private community where only the people you want to see your content can see it, then you need to control the relay where your content is accessible. There is no other way on Nostr.