One interesting idea that I heard recently for stopping spam was to have everyone respond to every spam message they got. This would overload the spammers systems and make it very unprofitable for them.
The only method I have seen that would stop you from getting spam is to set up your mail server or client to drop any mail from an address not in your address book. The user would have to enter someone's email address before they received any email though which could get to be a pain.
Too bad we couldn't round up all of the spammers and make them pay for what they're doing to the internet. Maybe send them to war ravaged countries to clear mines.
goates