On the facility side...you can charge a small ER fee as most patients have at least been triaged. You use the diagnosis of the presenting problem when billing.
On the physician side...you can not charge, if the physician never saw the patient. If the physician did see the patient and the patient left before being discharged, you may be able to charge based on the documentation that was done by the provider. Make sure they notate that the patient left before final disposition and again use the diagnosis of the presenting problem when billing.
If the patient was not seen by the provider, you should still put in a "ER Walk" no fee charge for tracking purposes. Most ER's like to keep stats on patients that leave before being seen, so they can improve on their ER wait times. Hope this helps a little...thanks.