Additional Information
You may also want to delete any associated auth mappings through the web UI, just to follow the general security principle of least privilege. Also be aware that if the same user is added back to the server later, any existing auth mappings may be re-associated, which may or may not be your desired result.
If you delete the user, but they're still part of the Active Directory group mapped by
Single Sign-On, the entry will just be created again the next time that user logs in.