I'm noticing more and more spammers are getting their own ISP. So, when we report spam, they directly receive the spam complain, and it just confirm that the email address works, and the spam never stops.
You visit the ISP web site, it seems legit, but there is no real contact information or physical address.
They also sometime uses Gmail as the abuse address, which doesn't seems normal for a true ISP...
So, my suggestion would be:
As well as reporting to the sender ISP, optionally report to the sender ISP ISP. So, the provider of the ISP will also get noticed of those spams.
I've done this once or twice in the last year, and the spam ceased immediately. I think the spammers had a hard time getting back from this major shutdown.
Sure, the function might be available to advanced users, but this would help a lot...