I am building a multisite network of blogs with buddypress

Hello,
I am building a multisite network of blogs with buddypress installed on main site.
I am learning by doing and so far, I can see blogs posts and coments fueled into the buddypress activity stream which is nice.
However, my users are seeing all activity on ablogs he will recieve all sites of the network even blogs or sites they are not yet suscribers...
Is it the way buddypress multisite is build ?
Is there an option to let users decide what blogs posts/forums posts and comments will be included in their own activity streams ?
If not, is there a plugin or parameter to set, to make users receiving only the blogs they are part of ?
Best regards,
Hervé