BuddyPress.org

It’s possible, you’d have to check the current blog and the activity type before the activity is saved to the DB.

Apply a filter to “bp_activity_type_before_save” (located in bp-activity/bp-activity-classes.php), check to see if the activity is a blog post or a blog comment.
Then, check the global $current_blog variable to see if it matches the root blog only.

That should give you a few pointers to run with.