add_filter('woocommerce_available_payment_gateways', 'applied_coupons_hide_payment_gateways', 20, 1 );
function applied_coupons_hide_payment_gateways( $available_gateways){
    // Not in backend (admin)
    if( is_admin() ) 
        return $available_gateways;

    // If at least a coupon is applied
    if( sizeof( WC()->cart->get_applied_coupons() ) > 0 ){
        // Loop through payment gateways
        foreach ( $available_gateways as $gateway_id => $gateway ) {
            // Remove all payment gateways except BACS (Bank Wire)
            if( $gateway_id != 'bacs' )
                unset($available_gateways[$gateway_id]);
        }
    }

    return $available_gateways;
}