BuddyPress.org

Go to your BuddyPress settings and set “Disable activity stream commenting on blog and forum posts?” to yes. That way they can comment on the activity stream, but not for forum posts where you want them to click through to the forum post and comment there.

Probably long term it would be cool to have the Reply button on forum/blog posts just go to the forum/blog post comment area. I know this bifurcation of commenting has been discussed a number of times.