ACP --> vBulletin Options --> Solved Threads --> Excluded Forums
You simply enter the forum IDS that you don't want having the the "Mark threads Solved" option.
You can find the forumID by going to ACP --> Forum Manager --> Then edit a forum and you'll see the forumID at the top.
EDIT: Do you mean the "Mark threads Solved" under thread tools being bold?
Regards Jason