I agree @james_whalley that directions need to be clear and students can use this as a learning opportunity when they don't follow directions. I'm sure my faculty member feels she has already tweaked her directions to get most students to follow them, but is getting tired of having a conversation with a few students every semester that don't follow those directions.
I would like to point out that Canvas has already provided an entire setting to disable attachments for Discussions. I think the fact that students can still use the Document Upload button to attach a file defeats the setting to disable attachments. Canvas went through the effort to create that setting; I went through the effort to find it; the teacher disabled attachments, and now students are still attaching files. Little bit of an oversight... you can see why I'm hesitant to push this back onto the instructor. From her perspective, the setting is broken, right?