Exclude certain forums from search results

[Xiaodidi8]

Hello, 

is it possible to exclude certain forums from the search results? For example i have an "Off topic" forum and i don't really to fill up the search results with this content. So I am wondering if there is a way how I can define the searchable content? The only option i found so far is to exclude content by the last update time but not by forum.

Does anybody has an idea?

Thanks, Robby