2 BadgerDog:
For specific usergroup:
ACP ->
Forums & Moderators ->
Forum Permissionst ->
Edit permission for that usergroup in that forum where you want block search. And set:
Can Search Forums -
No
For specific sub-forum for all of the users
ACP ->
Forums & Moderators ->
Forum Manager -> Edit specific forum and set:
Index New Posts in Search Engine -
No
(Setting this to 'No' will prevent anyone from being able to search in this forum) and
rebuild search indexes:
Maintenance ->
Update Counters ->
Rebuild Search Index
This will take some time! Depending how big your forum is. And i recommand to CLOSE forum before you start this !!! And, ofcourse, make a DB backup