Actually, it is "that simple of a business" -- shut down every single one that steals addresses, buys stolen addresses, provides incorrect return addresses, or uses open relays.
My spam filters (thank you, EFM!) stop between 75-100 pieces of spam per day. Not a single one was "opted-in." They use every kind of trick to avoid filtering. Munged return e-mail addresses. Use of encoding and forwarding to obscure the "payoff" website address. Subject lines designed to look like it's just a message from your friend "Pete."
The thing that burns my britches about 99.9% of spammers is that they spew a mile a minute about how what they're doing is a perfectly legitimate business, all the while committing outright fraud in order to beat perfectly legitimate filtering:
Scelson, who designed the software, says it will penetrate virtually any system designed to stop ads from reaching the intended mailbox.