Hi @TahsinGökçeer,
I'm not sure that I have an exact solution for you, but I do have an observation that could be playing into this issue. When I'm logged in to the Cavas instance for my institution, I just see the "delete from <institution_name>" link, not the "delete from all" one as well. I'm assuming your Canvas instance may be part of a trust relationship with other institutions, could you potentially confirm that? If that is so, I wonder if that user's "home" instance is yours, but that they are also enrolled in courses from trusted institutions with that account too. In that case, if you deleted the user from your instance, they'd not be able to access the other courses either, so Canvas *might* be preventing that on the back-end. I'd say it would be better to not even show the links in that case, but trusts get very complex, so this could be an oversight. I could also be completely wrong with my guess here.
Let us know if the scenario I laid out above might be right, or if I'm totally off base. Depending on your response, someone else might be able to offer alternate suggestions here.
-Chris
