There’s a plugin for this:
https://wordpress.org/extend/plugins/buddypress-block-activity-stream-types/
It’s an older plugin and the developer is now busy with other projects, so you’ll need to update the plugin manually so the admin menu will show up in WP.
Read this tutorial:
http://code.ipstenu.org/2011/wordpress-3-1-network-menu/
(Edit) Just read the full post, you want to hide it from group feeds only? Or throughout the site? If throughout the site, the plugin will do the job. If only on group feeds, you can take the basic premise from the plugin, then you’ll need to do a conditional to check if you’re in a BP feed.
