From their FAQ (http://www.sparkpeople.com/community/help_answer.asp?id=14)
Click on ‘Start’ in the top left corner of the screen.
You’ll now see your Start page.
Click on the ‘Account/E-Mail Preferences’ link in the upper left corner of the page.
(Note: Your Account/E-Mail Preferences link can also be found by rolling your mouse over your username in the upper right corner of the page and selecting Account/E-Mail Preferences in that menu.)
Scroll down the page to the ‘Close Account’ section.
Click on ‘Close Your Account.’
Click on ‘Yes, I Am Sure.’
Your account will now be closed.