MTN cant fix address change on invoice.

Me and my wife are with MTN from 1996. We have sice 2002 moved 3 times but our very old address up to 2011 still reflects on our accounts, not withstanding given poa in at various MTN stores over the yeras. My wife went to George MTN to change address last year(Ferbruary) went again and again, phoning, but to no avail. Send e mails of which we have proof of. Send 52 pincodes to us to try to lock a query, but no change. The staff says everytime that it will be correct on the next invoice. But the address on the invoice still wrong.