By the way, I set a disk space limit (2 GB) in the customer panel (but no limit in website panel) but a user use 4 GB, how is it possible?
Limits are applied on website basis, not client basis. So basically you have not set a quota at the moment. The limits in the client settings are the max quota that a client can assign to its websites when he creates or edits a site. As admin you can override this limits and thats what you did in your case.
To fix that, edit the website settings and set a quota there for the site.