It's definitely not what my host does, and they've got something like four trillion hosted domains. (I forget the real number, but it has more zeros than you would think possible.) Very very rarely a visitor gets hit with a 503 if it is an IP belonging to a known evil robot. They've also got something in their config file that looks for certain patterns.
But in general it's up to the individual user who they want to block. Lots of robots are annoying and troublesome but hardly any of them are actively malign. And most of the really nasty ones keep skipping to a new IP so you can't block them in advance. As far as I know, I've never got a 406 error. Not even with robots who come looking for nonexistent php pages with suspicious queries. I had to go look up 406 just to find out what it is.
When I first got this host, I was told that I wouldn't be able to send e-mail from my domain. That is, I can send it just fine. But the recipient may not get it, because a lot of ISPs block e-mail coming from anything that isn't another ISP. That's probably fairly common. But that's e-mail. Wholesale blocking of every IP under the sun seems extreme. Even the Ukraine sends human visitors sometimes. Are you sure you have the best possible host? :)
Oh, and yes, Comcast is a major ISP. They may do servers on the side, but I think their main business is with humans. I definitely would never think of blocking them wherever I see them.