You can cancel your account under the My Account section.Go to: Login » My Account (Top Right Corner)
On the popup, please select the reason why you are cancelling the account, then click on I understand that by cancelling my master account, all other accounts associated with this account will be automatically removed. If you would like to retain your surevys and data, click on Retain all surveys and data » Cancel Account
If you log in and receive a message regarding the account being blocked, you should see the Cancel/Delete Account link on the left. You can cancel your account by clicking on this link.