function payment_gateway_disable_country( $available_gateways ) {
global $woocommerce;
if ( is_admin() ) return;
if ( isset( $available_gateways['authorize'] ) && $woocommerce->customer->get_billing_country() <> 'US' ) {
unset( $available_gateways['authorize'] );
} else if ( isset( $available_gateways['paypal'] ) && $woocommerce->customer->get_billing_country() == 'US' ) {
unset( $available_gateways['paypal'] );
}
return $available_gateways;
}

add_filter( 'woocommerce_available_payment_gateways', 'payment_gateway_disable_country' );