If you want to view who the top 100 message senders on your site is by ID number of the sender, run the below query in phpMyAdmin. It will be helpful to find spammers.
SELECT Sender, COUNT(Sender) as MessageSent FROM Messages GROUP BY Sender ORDER BY MessageSent DESC LIMIT 100;
You can look at a sample of their messages by running the below query (where 7701 is the Sender ID ):
SELECT * FROM `Messages` WHERE `Sender` =7701 LIMIT 30;