In order to limit the number of emails each domain's account can send per hour, you will have to login to WHM in order to make this change. You will need to use the root user to login to WHM. The address to login to WHM is: http://serversip/whm (serversip would be your dedicated or vps servers ip address)
Once your logged into WHM, browser over to:
Server Configuration >> Tweak Settings
Scroll down on the "Tweak Settings" page, and look for the following: "The maximum each domain can send out per hour (0 is unlimited):"
The default value is "0" which means unlimited. You can change the value here to whichever value you are looking to change it to. For example: "100" will allow domains only 100 emails sent per hour.