In vB3, this was a matter of adding '&exclude=x' at the end of the search url.

In vB4, this results in an error as follows:

Fatal error: Call to undefined method vB_Search_Criteria::add_excludeforum_filter() in /home/xxx/public_html/search.php on line 1056

I've also tried doing it via plugin, but can;t seem to get $vbulletin->GPC['exclude'] .= ',76'; to do anything in any hook location.

search_getnew_start doesn't seem to exist any more.