This would work if all email went through the respective ISP's servers. Almost all legitimate emails do. Most spammers abuse insecure systems (usually in foreign countries such as Korea) which relay emails for anyone (who knows a few technical details and how to find these open servers), so spammers are already going around the ISP that would be charging for email. They've been doing this "relay rape" for years. One problem with charging is if you charge for every recipient message generated by your sending a message (which would be how it would have to work to make spam pay its own way), mailing lists would no longer be viable. I'm on a couple of lists with over 500 subscribers. A single post to such a list would then cost five dollars. Usenet operates differently, but probably has as many security holes in it as email. Half the people on the Internet don't know what Usenet or newsgroups are, and ISTR that only 20 percent of people online ever read or post to newsgroups, so unless there's an easy fix that gets implemented, I have less hope that anything can or will be done to stop Usenet spam, and I'm not that hopeful about stopping email spam.
Yet another link with antispam information:
I subscribed to spam-l in 1997, thinking that the spam problem would be essentially solved in a year or two. I was so naive...