Login to SIS as the user you are trying to delete.  Navigate to Storage Access on the left-hand menu.  On the right-hand panel you will see the storage system in question.  Delete it there.  Then you can log back in as admin and delete the user.
I am trying to delete SIS user from VSI appliance but it gives me an error "Failed to delete user: . Details: Can not delete a storage administrator: who still owns storage systems: APM0014376499 APM00143764112 "   For sure before doing that I unregistered those storage devices in Web Client and even unregistered  Solution Integration Services as well.  Any ideas?
Cannot delete SIS user from VSI 7.2