Hanna said Spamhaus experienced its first denial-of-service attack in 2003.
"This has been the biggest for us," he said, "but certainly not the first one."
Cloudflare's Prince said denying access to a website through cyberattacks is the truest assault on Web freedom.
"Our role is to allow the internet to achieve what it aspires to -- that anyone, anywhere can publish any piece of information and make it accessible to anyone, anywhere else in the world," he said. "It's blatant censorship.
"Whether Spamhaus is a good organization or a bad organization is irrelevant to me. We protect American financial institutions, which some people think are evil, and we protect WikiLeaks, which some people think are evil."