This feature gives the option to regulate each role's message composing permissions.
Select the Messages too from the Tool Menu in your site.
Permissions is located below the Course Site title.
Permissions are assigned on a role-by-role basis. For instance, to prevent a student from sending a message to "All Participants", un-check the Student Permission box located next to the option, "Allow the 'All Participants' options to the 'To' field."
Note: These options will be visible when clicking the "To" text box in Compose a Message, within the Message tool.
Click Save or Cancel to quit.