Traffic from ISPs who refuse to play by the rules will be refused by the
Internet Community.
In other words, they will be excommunicated.
I that is demonstrably not true. The system has gotten too big for
that, and the need for connectivity too large. Blacklisting is no
longer a sufficient threat, and attempts to do so to mixed-use
providers have resulted in law-suits (witness MAPS). The entire
thing is complicated by the subjective nature of spam, and the
question of when to pull the trigger on an ISP. Are they spam
friendly, or just having trouble keeping their system clean?