If you turn off the option, and it was previously on, then it simply stops any new posts from being indexed for searching - it doesn't remove the indexing that was done for previous posts. I believe you would have to Rebuild your Search Index if you want it to wipe away the indexing for the old posts.
If this is set to No, posts made in this forum from this point on will not be indexed by the search engine. Such posts will not able to be searched for. This is desirable if you have a "Testing" type forum, or a forum that is chit-chat that you don't want indexed.
Ok I turned it back off and will see what happens thanks
Turning off Indexing for them is different than simply removing them from the New Posts search. Turning off Indexing means that if you do a search for the word "yyyy" in posts, any posts that aren't indexed won't be searched for that word. But, New Posts is different. If you don't want a forum to show up in New Posts, then just add "&exclude=xx" to the end of the url the template (usually the navbar template). Or, I think there is also a Mod that uses a plugin to exclude certain forumids from the New Posts search.