Just running into this issue. Looks like people have been asking for some sort of inbox block feature (at any level) for years. I get that there are technical reasons why this is difficult, but this should be a priority. In my case a student is claiming harassment and we can't turn off inbox privileges for individual users or block users in inbox. I just want to suspend this feature until the investigation is made. Is our only choice to stop this behavior to drop the student sending unwanted messages?
Please tell me there is some workaround. I tried creating a separate section in the course for just one student, but that doesn't stop it.
FYI: The only solution was, like OP said, to create a new role based on Student but turn off Conversations in role permissions. The student can still message the instructor but they can't message any other students in the course. The admin would have to create the Role, then change the Student who is sending inappropriate messages to that Role in the course. Here's a screen shot of what that would look like with a new role "Probation"
