Mmmm.
My gut instinct tells me that you can PROBABLY alter the foreach loop at around line 634 of ubb_lib_files.cgi to make it skip the forum in question.
However, be warned that this my have unintended side effects. The TAT system is closely tied in with the content island system...
(Also, this change won't be immediate. It may take removing the TAT datafile (cache-/search_index/active_topics.cgi) and a cache clear.)
