function payment_gateway_disable_total_amount( $available_gateways ) {
global $woocommerce;
if ( isset( $available_gateways['paypal'] ) && $woocommerce->cart->total == 0 ) {
    unset(  $available_gateways['paypal'] );
}
    return $available_gateways;
}
add_filter( 'woocommerce_available_payment_gateways', 'payment_gateway_disable_total_amount' );