[Lynne]

No need for paid mods, play with this (for fulltext searching only - I use a much more involved version on my site and it works - you will need to rebuild similar threads for it though):
hook_location - search_similarthreads_fulltext

PHP Code:

$hook_query_where .= " AND thread.forumid NOT IN ('x','y','z')"; 


I think I had to do it a bit differntly when we weren't using fulltext search, but I no longer have the code I used then.