ISPs are the problem

ISPs could be the solution. Most include some variety of modem device in the package. Make it incorporate an SPI firewall. Make it block SMTP traffic to any host other than the ISP's mailservers.

But no, a disposable USB-attached piece of shit is the norm.

Complain to ISPs whose subscribers relay spam. Currently they simply don't feel any pain. We should be overwhelming their abuse mailboxes in precisely the same ratio that they are instrumental in flooding our own.