Immediate reset when all nodes for a VS are unavailable

Which profile are you using? For TCP and FastHTTP, there is no way to do this, as these profiles offload the handshake and proxy the traffic; by the time the LB pick is made, the handshake has already completed. If you're using fastL4 it might be do-able, since I believe (without looking at the code) that an lb-failed event with fastL4 triggers a RST to the client.