Wouldn't work. They'd just move their servers to Canada or the Caribbean.
The only solution that would even slightly work would be to extend the "junk fax" law to cover spam as well. That would, of course, have the same problem as your suggestion, but at least it would FORCE the people involved to move out of the US or face unlimited class-action lawsuits, and also force (if written correctly) all major Net backbones to cut off access to the IP addresses of all spammers no matter where in the world they come from.