They could install software at the router level to identify spam and silently drop it. That would fix most of the problem because once the spammers know their messages aren't getting through they'll abandon the tactic.

Another, more controversial idea would be to prosecute those who respond to spam ads. If it weren't for Joe Smhoe buying their penis enlargement cream (aka, 'Crisco shortning') they wouldn't spam. Send out some bogus email ads and fine anyone who actually orders the product. Before too long the spammers would find their response rates dropping from 1% to 0.001% and they'd go out of business.