What's needed is a means by which they can be forced to pay for it. If we pass legislation which enables the restriction of email in general - even as an unintended consequence - we are going to regret it.
Consequently, I'd rather see this handled as a civil matter involving loss of revenue. The Government needs to use its authority to assist ISPs and others in suing spammers to recover lost revenues, etc. - whether they're foreign or domestic. (The U.S. government without question has enough influence with foreign governments to force them to go after spammers outside our borders - if it wants to use it).
But don't expect the rocket scientists in Clowngress to understand any of that (or even care). They'll just seize the opportunity to legislate to expand the power of government to regulate, no matter where, or why - and no matter what the unintended consequences.