BP Messaging Control is a plugin that offers very powerful controls over BuddyPress messaging via a simple settings interface. The plugin maps out all the roles used on the site and allows each to be assigned No Messaging, Admin Only, Reply Only or Full Messaging. In addition email quotas can be assigned. This is ideal for monetizing or differentiating messaging as part of your membership site.
BuddyPress Restrict Messages is a useful plugin for sites that wish to control in-site spam to users. It comes with two modes of operation – Admin control and User control, these allow users or site admin to restrict messaging to friends, followers or just to members of the same groups.