To me that's more of an argument for GP's idea. Make it rational for them to care about it. Display a big "You can't visit this website because <ISP name> won't

To me that's more of an argument for GP's idea. Make it rational for them to care about it. Display a big "You can't visit this website because <ISP name> won't let you. Here's their email:" Obviously it's not worth it for any website that matters to actually do this, but it's fun to think about.