Use the isAvailable method for the payment method in question, E.g. for Mage_Payment_Model_Method_Checkmo:
public function isAvailable($quote = null) { return $_SERVER['REMOTE_ADDR'] == 'IP.ADDRESS'; }
Use the isAvailable method for the payment method in question, E.g. for Mage_Payment_Model_Method_Checkmo:
public function isAvailable($quote = null) { return $_SERVER['REMOTE_ADDR'] == 'IP.ADDRESS'; }