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Like Twitter, the <a href="https://wordpress.org/extend/plugins/buddypress-followers/" rel="ugc">BuddyPress Followers</a> plugin adds the ability to “follow” users on your BuddyPress site.
However, by default, any member on your BuddyPress site is allowed to send private messages to anyone in your userbase.
This plugin allows members to send private messages only if the recipient is following them. This will help prevent private messaging spam on your BuddyPress site.
<h3>Recommendations</h3>
I do not recommend using the BuddyPress Followers plugin in tandem with the BuddyPress Friends component.
If you’re looking for a similar plugin for the BuddyPress friends component only, check out <a href="https://wordpress.org/extend/plugins/buddypress-private-message-for-friends-only/" rel="ugc">this plugin</a> instead.
<h3>Donate!</h3>
I’m a forum moderator on the buddypress.org forums. I spend a lot of my free time helping people – pro bono!
If you downloaded this plugin and like it, please:
<ul>
<li><a href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=S6EZYK894S99G" rel="nofollow ugc">Fund my work soundtrack!</a> Music helps me help you! A dollar lets me buy a new tune off Amie Street or emusic.com! Or if you’re feeling generous, you can help me buy a whole CD! If you choose to donate, let me know what songs or which CD you want me to listen to! 🙂</li>
<li>Rate this plugin</li>
<li>Spread the gospel of BuddyPress</li>
</ul>

BuddyPress Private Messages for Followers Only

Allow members to send private messages only if the recipient is following them.  Requires the BuddyPress Followers plugin.

rich! @ etiviti

rich

This plugin is perfect for private message spam prevention. It allows site admin and users themselves to limit exactly who can send messages to the user.
This plugin creates an admin settings page called Restrict Messages. Here the site admin can choose to impose global restrictions over who can send messages to the user.
If message restrictions are delegated to the user, a user settings page is created at Profile>>Settings>>Messages.
Each of these settings pages, depending on which is active, allows private messaging to be restricted to:
<ul>
<li>Friend Connections</li>
<li>Follow Connections</li>
<li>Groups Connections – with further selection of public, private or hidden status levels.</li>
</ul>
The public setting allows connections from all groups to send messages to the user, otherwise private or public stipulate only messages from these types of groups.
Once a restriction has been implements, the plugin removes the private message from the users profile, removes the usename from the compose message send to auto-complete and rejects any messages send where the send to includes an excluded user.
This plugin was originally created by Rich @etivite and is now supported by Venutius.
<h3>Extra Configuration</h3>

BuddyPress Restrict Messages

Exactly what I needed to improve a situation on a client site's, and lovely to see @venutius supporting and maintaining this plugin! Thank you!

Does the job!

This plugin allows the site admin to restrict who can send private messages or to enable the users to choose themselves.

BuddyPress Private Messages for Followers Only vs BuddyPress Restrict Messages