How to exclude certain vBulletin sub-forums/sections from search results

In admin area, Forums & Moderators, Forum manager, Edit forum, is option: Index New Posts in Search Engine
when disabled, vBulletin will stop indexing posts/topics added to this forum. So it will not appear in the search results of the vBulletin.

Old content of these forbidden forums will continue to appear in search results until one empty search index and rebuild it again from within: AdminCP, Maintenance, General Update Tools, Rebuild Search Index. There is a link "Click Here to empty the search", after emptying, rebuild index and enjoy searching without results from the forbidden/excluded forums.

EXCLUDE FROM "New Posts" page too
See here to exlude from New Posts page: