Since lots of (windows) computers are compromised to function as open mail relays it is simple enough to block port 25 and stop the relays from functionning.

In any case, laws are not very effective when the individuals hide their identities as routine such as when sending spam.

However, if the advertised products in unsollicited bulk email would be liable for the spam, then it might be effective. Of course, you would also need to show a link between the spammer and spammed product, otherwise it might turn out to be a defacing scheme, and that might prove too difficult to implement.

The most realistic solution is probably to require that all mail senders be registered in dns records (as in Sender Policy Framework) AND hold them responsible for unsollicited bulk email if they do not put a stop to it when it is reported.

One way to hold them responsible could be to sue them in courts (costly, lengthy and not very practical).

Or to ban the offending servers on a public list. Then everyone blocks those servers.
No more spam.