Does anyone know anything about rules governing Dr's

The Dr I had, refused calling in a new script for a new pharmacy that supplies my meds thru Schering.  Reason is I owe him $225.  had been $575 or thereabouts.  He dumped me as a patient in Feb. I think was Feb.  Not really sure because I found out from a local pharmacy when they called his office for a refill on Diabetes script.  I was told I have to pay the $225 off for script to be faxed in.  Oh, and when I called after Pharmacy told me about his office saying I was no longer a patient, they said they sent me a registered letter that I could come by and get a copy (I didn't).  Don't you have to sign for them?  I never received one, first off, if I knew I would not have had pharmacy call there.  This all took place in the great state of GA.  Wish I was still at home in FL.
Connie