Filters will never solve the spam problem.
And there is one angle in particular that is available for stopping spam:
But what you are proposing is effectively just another type of filter. It's something that will reduce--but not eliminate--spam, and is something that eventually the botnet folks will figure ways to get around. If you think that spam filters will never work, then increasingly stringent regulation of domain registration will not work either.
The parent's point was that removing the incentive amounts to removing the profit motive, and this is essentially impossible. Your suggestion about `the damned registrars' does nothing to remove the profit motive.
"Engineering meets art in the parking lot and things explode." -- Garry Peterson, about Survival Research Labs