> > if Google implemented my suggestion they could never be blacklisted

> Google seems to have built their brand intentionally to be the opposite of what you're asking for though; and absolutely they could be blacklisted with a simple "GMAIL ADDRESSES NO LONGER ACCEPTED HERE".

I think "you can't block GMail" here is meant in the sense that "you can't block the Google crawler". It's certainly technically trivial to do so, but the opportunity cost from lost users will be, for most businesses, unacceptably high.