When did I say Spamhaus should go away?
Anyway, you're missing my point: Why on earth do you have to blacklist the entire ISP to deal with one customer? Presumably if you are certain that one specific customer is to blame, and you're demanding that customer be disconnected, you know which IPs the customer is using and you can block them. Or you are otherwise able to identify the customer, say through headers - in which case, you should filter those headers. Blacklisting the ISP should only be necessary if the ISP is intentionally helping its customer obscure their identity and disguising spammy/malicious traffic as normal traffic.
There is no excuse for punishing innocent web users and businesses for the actions of third parties, or for holding those innocents hostage in order to force their ISP to do something you want. The fact that the ISP later complained to the authorities suggests to me that this is exactly what happened. Is it really impossible from a technical perspective to do anything other than blackhole the entire ISP? I'll admit I'm not an expert on IPv4, but that seems implausible to me.
The problem with these policies and principles you're defending is that they can be equally applied to innocents in order to do harm. I'm not claiming I have some perfect alternative; I am merely pointing out that Spamhaus' means don't seem legitimate, even if their ends are legitimate. And in this case, the legitimacy of the ends can come into question as well.
There's a gray area between obvious spam (penis pills, etc) and obvious solicited mail (personal mails between individuals) - Twitter sends me emails regularly containing tweets by people I follow, because I have an account, but I never solicited that email. Is it spam? What if there are other people who do want those emails, even if I don't? Is it really appropriate to blacklist Twitter?
The Twitter example isn't a fantasy, either; Gmail automatically started sending those things to my Spam folder without my intervention, and I've seen it do that to other content in the past. Would you be happy if Spamhaus added the IP ranges of businesses and ISPs you interact with to its blacklists? What if they blacklisted your home ISP?