Tragedy of the digital commons

The internet is a network of networks, decentralized and generally unregulated– a digital commons. Is the internet suffering a tragedy of the commons?
Wikipedia has a concise summary of the evolution of the spam problem on the net: Spam. First, Usenet fell to the assault of spam. Then, e-mail. Now, the web is under assault.
Matt Haughey noticed how spammers are abusing Google’s Blogspot service: When it rains, it pours:

It looks like one monster spam blogger has unleashed a boatload of new blogspot blogs, always in the form of keyword-(random number).blogspot.com (like lottery-123123.blogspot.com). They suck in RSS feeds from blogs like mine and boingboing and others, then insert random phrases into the copy, with a link to their own sites using phrases they want to game google with

Mark Cuban: The Shit hit the fan today: “The blogosphere was hit by a blogspot.com splogbomb. Someone did the inevitable and wrote a script that created blog after blog and post after post.”
Are all open digital communities subject to the spam dilemma? How do we bar unproductive uses of the digital commons without raising the barrier to entry and blocking other voices?
The internet revolutionizes communications, making it possible for anyone to reach a global audience. That has a downside.