Customize Blog Search index scope

Hi! I have 3 blogs on my site and a main Search control that returns all the post from all the blogs of my site. I want to filter out the posts of one of these blogs so they don´t appear on the search results.
Is there any way to customize the search scope? I checked in the "Search and Indexes" (see the image attached) option but it only let me include all the blogs or none, I can´t select an specific blog.
I´m using Sitefinity 4.2.1733.0,
Thanks for your help.

Iván.