I'm all for fighting back. Check the headers of the email and send it back where it came from (if you can). You'll find their ISPs have an acceptable use policy similar to NTLs which, if you read it, says they're not allowed to do that.
I realise this isn't possible for all of them but I think the attitude of just using spam filters ensures that problem continues to linger. If you think about it, they are actually stealing our bandwidth. Any ISP that receives a message regarding 'theft' isn't going to ignore it.
Hey if NTL can consider uncapping your modem as bandwidth theft then I can regard spam the same.
