If only ISPs would actually crack down on (usually unwitting) users whose systems are participating in a malicious botnet or otherwise have their networks compromised. They could offer temporary disconnection + anti-malware support to get the user cleaned up, if they actually gave a shit.

No residential last mile broadband ISP at the scale of hundreds of thousands or millions of customers is presently willing to spend the salary/benefits/fully loaded employee cost on the massive teams of (somewhat technically clued in, not low wage) people it would take to effectively implement this.