Canvas is very limited in this regard. The ability to disable students messaging each other can only be done at the 'role' level (i.e. student, Teacher, TA, etc.), which is applied to the whole system. In other words, if conversations is off for the 'student' role, it will affect ALL students in Delta's Canvas!! Every class. Everyone!
There is a "feature idea" in Instructure's forum that was submitted back in 2018 to allow control of disabling conversations among students at the course level. It has not gotten much traction. 10 votes. I think the primary reason for this is because the In-box serves as one of the primary ways to ensure 'student-to-student' interaction in online courses, which is required for regular effective contact.