Accounting for nonexistent users / NAS ?

Hi all, we are using freeradius with mysql. Accounting works fine, but we discovered that the server is doing accounting for users which don't exist at all in our system. They are probably local users but accounting information is sent to our servers. How do we allow accounting for only our users? The server should check whether the user is in the radcheck table as we only keep users that which have a valid account. How can this be achieved? Furthermore, it would be good if we could verify the NAS value so the server should only do accounting when the NAS is allowed. Thanks for the answers.