Determine who site participants can send a message to
This feature gives the option to regulate each role's message composing permissions.
Go to Messages
Select the Messages tool from the Tool Menu in your site.
Permissions is located below the Course Site title.
Check the corresponding boxes for desired permissions
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.