If you read the article, you'll find that he's using multiple servers all over the world.

And many, many hijacked open relays, and open proxy servers, In most cases, you'd only be attacking computers that don't have anything to do with the spammer.

Most of these spammers are their own ISP.

Well...That's not fully true, they have to get their internet connection from somewhere, and that means a connection with at least one other ISP.
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--Ben
78GB MkIIa, Dead tuner.