It doesn't have to be enforced. As long as a reasonable percentage of hash rate is following that policy an attacker that tries to flood the network will fail to prevent normal transaction traffic from going through and will just end up transferring some wealth to the miners.

Although the existing default mining policy (which it seems about 70% of hashpower follows) of setting aside some space for high-priority transactions regardless of fee might also be enough to cause this attack to fail in practice.