It will be great if Canvas create on the Status field of the Users file a new value = "Inactive".
On the log-in page, if not using any auth. provider, a error message will pop-up to alert the user that his credentials are "inactive" or any other kind of message like this.
Even if the user has active enrollments, he won't show on the tools like: people, grades, reports etc.. simply because his Account status is inactive.
The reason why i'm creating this idea it's because if we delete a User and then active the user (with same credentials) all the enrollments that he had won't return (only if we enroll him again).
Edit: Please support this idea Disable/Deactivate User at Account Level as mentioned below.