Lets say each website / server admin blacklisted various ISPs from their servers because they did something the admin didn't like. Maybe they noticed throttling to/from their site, or spam, or just had such a bad experience once as their customer that they now considered that ISP to be "bad for the internet".

What would we end up with? Most of the internet not working for anybody. Friends sending you links that you can't get to because you're on different ISPs. Needing to buy service at more than one ISP to "provide coverage". If it happened on a large enough scale, it would unravel what we know as the internet.

This is what I mean when I say it's a bad idea.