You stop peering with providers originating spoofed traffic. The whole point is that you don't know which host is doing it (the IP is spoofed!). All you know is

You stop peering with providers originating spoofed traffic. The whole point is that you don't know which host is doing it (the IP is spoofed!). All you know is that a provider is allowing spoofed packets (with a probe from participating endpoints in a measuring service). That provider should be dropped and everyone on that provider should suffer collateral damage because that's the only way to economically incentivize them.