Many factors motivate spammers, but a principal reason for spamming is to make money, directly or indirectly. In this sense, it is not at all the source of the spam that is at issue, but rather the sink of the money. If illegal spam promotes the sale of goods or services, and there is a venue to place the money, a legal system can seize the money, punish the offenders and provide at least some disincentive.
True, there will always be people who care not about the law, or will find ways to skirt it. But even a mild amount of friction can significantly reduce the mass of spam. The trick is not to make spam free to send. Just a little friction can make a lot of difference.
True, there will always be risk of first amendment and free speech violations in spam regulation. Bit it need not be so -- effective spam regulation can be obtained merely by making it illegal to lie about the spam. Consider one modest proposal.