If you want to disable commenting only for forum topics and blog posts in activity stream, then you only need to go to wp-admin menu BuddyPress > Settings and click Yes radio button beside “Disable activity stream commenting on blog and forum posts?”
If you want to totally disable commenting in activity streams and you have a bp-default child theme, copy over https://buddypress.trac.wordpress.org/browser/tags/1.5.2/bp-themes/bp-default/activity/entry.php to your child theme folder following directory structure like mychildthemefolder/activity/entry.php then delete lines 54 through 84 as seen in link above. Save file and upload to server.
If you used the BP Template Pack to make your WP theme compatible with BP, open up /activity/entry.php which had been transferred to your WP theme in server during the compatibility process and delete lines as stated above.
