[kh99]

If you edit the forums in the forum manager, you can set "Index New Posts in Search Engine" to No. That will make it so that new posts can't be found by a search. There's probably a way to remove the old posts from the search index by rebuilding it or something, I'm not sure (and it may be a major operation if your forum has a lot of posts).

There's also an "exclude" parameter that can be used to exclude certain forums from the results, but it doesn't look like there's any way to use it from the search form. You could probably write a plugin to force certain forums to be added to the exclude parameter.

ETA: oh...rereading your post it sounds like you never want any forum posts returned? Then you can probably remove that result type from the search.