Take a look at this post: http://forums.b2evolution.net/viewtopic.php?t=3591&highlight=hide+blogs or do a search on the forums.

if( $blog == 1 )
{ // Special case: we aggregate all cats from all blogs
$this->request .= 'WHERE 1 ';
// hack to exclude specific blog cats from all blog aggregator
// note: category IDs are specific to blogs, so by restricting a cat you restrict the blog
$this->request .= 'AND cat_blog_ID <> 9 ';
$this->request .= 'AND cat_blog_ID <> 8 ';
$this->request .= 'AND cat_blog_ID <> 7 ';
}

Ok, sorry for my stupidness but how i must change the code to make it work on my site? how does this ID looks?