quan_flo

If there is a user (or a bot) that signed up at your site this user can start to send messages to every other user. 
I had some negative experiences with this and so I wrote a module that should help to block such a spam.
Buddypress Messages Spam Blocker introduces some restrictions to your users:
<ul>
<li>New users can send messages only 24h after their registration, so you’ll have time if bot registrations have to be removed manually (modify this value using filter ‘buddypress_messages_spamblocker_newMembersWaitingPeriod’)</li>
</ul>
Their are also some more restrictions for mass mailings (mails that are sent to “friends” of the contact list are not included in this calculation):
<ul>
<li>Users can send 6 messages maximum in 5 minutes (modify this value using filter ‘buddypress_messages_spamblocker_5m’)</li>
<li>Users can send 10 messages maximum in 10 minutes (modify this value using filter ‘buddypress_messages_spamblocker_10m’)</li>
<li>Users can send 20 messages maximum in 30 minutes (modify this value using filter ‘buddypress_messages_spamblocker_30m’)</li>
<li>Users can send 30 messages maximum in 60 minutes (modify this value using filter ‘buddypress_messages_spamblocker_60m’)</li>
<li>Users can send 35 messages maximum in 12 hours (modify this value using filter ‘buddypress_messages_spamblocker_12h’)</li>
<li>Users can send 45 messages maximum in 24 hours (modify this value using filter ‘buddypress_messages_spamblocker_24h’)</li>
<li>Users can send 50 messages maximum in 48 hours (modify this value using filter ‘buddypress_messages_spamblocker_48h’)</li>
</ul>
Users with the capability “edit_users” (admins etc.) have no restrictions for outgoing messages
Install, activate, and it will work.
More about me and my plugins
Since the year 1999 I do administration, customizing and programming for several forums, communities and social networks. In the year 2013 I switched from another PHP framework to WordPress. 
Because not all plugins I’d like to have exist already I wrote some own plugins and I think I’ll continue to do so.
If you have the scope at forums or social networks my other modules might also be interesting for you. <a href="https://wordpress.org/plugins/search.php?q=quan_flo" title="ifs-net / quan_flo WordPress Plugins" rel="ugc">Just take a look at my WordPress Profile to see all my Plugins.</a> Use them and if my work helps you to save time, earn money or just makes you happy feel free to donate – Thanks. The donation link can be found at the right sidebar next to this text.

Buddypress Messages Spam Blocker

This plugin will block mass mailing for the buddypress messaging system

rich! @ etiviti

rich

This plugin is perfect if you want to use messaging as a way of differentiating or monetizing your BP Site. It’s also good for controlling internal spam. You can control access to messaging and who the user can message. It also allows quotas to be set for each role and maximum message size. The options available are:
Based on each role you can disable or set limits on messaging.
Private Messaging:
<ul>
<li>Messaging Disabled – No access to messaging for this role.</li>
<li>Admin Only – The role can only message site admin.</li>
<li>Reply Only – The role can only message users who have previously sent them a message. Admin messages excluded from limitation.</li>
<li>Full Messaging – The role has unrestricted access to messaging.</li>
<li>Messaging quota; 1 to unlimited emails per month, week or day. Messages to admin not counted. </li>
<li>Message Character Limit: Enforce maximum number of characters per message.</li>
<li>Notification Email content length restrictions: Allows you to display only the first few words of the message so users will visit the site to view the message.</li>
</ul>
Public Messaging ( @Mentions and Activity Updates )
<ul>
<li>@Mentions Disabled – No access to public messaging for this role.</li>
<li>@Mentions Admin Only – The role can only public message site admin.</li>
<li>@Mentions Reply Only – The role can only public message users who have previously sent them a public message.</li>
<li>@Mentions all – The role has unrestricted access to pubic messaging.</li>
<li>@Mentions quota; 5, 10, 25, 50, 100, 250, 500, 1000, unlimited emails per month, week or day. Messages to admin not counted. </li>
<li>Activity Updates including Public Messages: Enforce maximum number of characters per Activity Update or Comment.</li>
<li>Notification Email content length restrictions: Allows you to display only the first few words of the message so users will visit the site to view the message.</li>
</ul>
Notification Emails
<ul>
<li>Set size limit for the usermessage token – the notificaton message content. This is useful to prevent the entire message being sent in the notification meaning users will need to visit the site to read the full message.</li>
<li>Enable Admin notification of user deletions.</li>
</ul>
This plugin needs BuddyPress to work and it supports both BP Legacy and BP Nouveau themes.
This plugin controls all aspects of BP messaging and makes sure the user only sees the messaging options they are entitled to.

BuddyPress Messaging Control

This plugin is a Swiss Army Knife for messaging, It allows the site admin to place restrictions on public and private messages including general rules …

Buddypress Messages Spam Blocker vs BuddyPress Messaging Control