n00b1 - 6:55 pm on May 3, 2012 (gmt 0)
By removing any useful content from there. To give an extreme example - return a 404. A manual redirect. Same way you'd move to a new domain without a 301. Google gives clear guidance and indication that the 'www' and 'non-www' versions of the website are treated as seperate entities and are ranked seperately just like different domains. A 301 is out of the question as it transfers everything (including link attributes and profile) but the other options aren't. And I wouldn't set anything as a preferred domain in GWT because I don't want link power to transfer.