Disabling bookmarks from the river doesn't sound like very generic need so I don't think there's a plugin for it.

I recommend creating your own plugin or paying someone to develop it for you. It wouldn't be complicated. You just need to register a plugin hook handler for the 'creating', 'river' hook. And then inside the handler check the type of the entity being added to the activity. If the type is "bookmarks" you just return false.