If someone has left your establishment and you need to remove their access to the CRM, marking them as "inactive" or "deactivated" will achieve this. This works regardless of their permission level.
You can do this from either the users' list:
Or from their profile:
Q. Who can make users inactive?
A. Only a User Admin can perform this action.
Q. Why can I only deactivate users - why can't I delete them?
A. This to ensure that the users actions are preserved in the platform logs and audit trails.