You enforce it by picking one or two high profile sites, and ruin the lives of the people who run it. Then everyone else "self complies".
That sure stopped illegal music sharing...... NOT!
Mischief of Anonymous Speech
"Anonymity tends to go hand in hand with mischief," he said. "Authentication tends to eliminate this mischief." I guess that is why federal campaign laws require independent expenditures, like political handbills, to have a disclaimer stating who paid for them.
Anonymity might promote the mischief of unseating incumbent politicians. In fact disallowing anonymity might have prevented America's founders from the mischief of creating a new nation! We should put an end to America's traditional mischief of anonymous political speech. King George would have liked the idea of stopping anonymous speech. He wanted every American printing press to have a unique number stamped on its platen, so every page could be traced to it's source.
FEC regulation of blogs could start the process by disallowing screen names .. or do you think that might chill participation?
That's how the IRS does it. And it won't necessarily be a site like this one. The reason is because There is enough of a network to spark revolt.