Why haven't black-hat hackers put the spammers out of business?

Factual Questions
1

Why haven’t the spammers and their clients been overwhelmed by denial of service attacks, repeated calls to their 800 number and the like? I would think that spontaneous self-regulation would have some effect. Why does spam persist?

Required Caveats:
Do not try this at home. Also, note that spammers may disguise their identity by including false return addresses. They may even send out spam purely for the purpose of provoking a response, then selling the live email address to another spammer. None of these considerations would phase a dedicated hacker or even semi-motivated amateur though.

References:
Any way to spam the spammers?(2/2003)
Understanding Spam (11/2002)
Death to Spam: A Guide to Dealing with Unwanted Email. “Death to Spam is an objective overview of unsolicited e-mail and the techniques available to defend your IN box. Learn about tracing mail, message filters, legal issues and more.” ©1997-2003
CNET, 3/1997: Hacker-spammer war intensifies
Infamous Spammer Spammed, Wired 1/2001

2

Most spam is relayed through improperly configured machines on otherwise innocent networks. DoSing these (admittedly negligent) people accomplishes nothing.

3

Going off what friedo said, there may or may not be hackers finding the compromised systems, killing the spambots, plugging the hole they came in through, and leaving a polite note. But of course, there are more than a few problems with this: not enough white-hats, too many compromised systems, and they’d still probably be committing a crime by doing that.

The law is an ass.

4

Point noted, gang. The actual web presence of the spammer may be small if he works through involuntary proxies.

Still, the spammers are usually selling something (or pretending to). One might think that the purveyors of super-di-ionistic water, herbal viagra, porn.com and low low low low low !low! rate mortgages would be susceptible to direct action, joe-jobbing notwithstanding.