Thanks to Yannick for the following instruction:
If you want to permanently delete your account, click here: http://www.tuenti.com/#m=Settings&func=view_delete_account_page
Select the reason why you want to permanently delete your account and click “Submit request for withdrawal”.
Other option is to deactivate: sign in Tuenti. Click on “My Account” at the top right of the screen. Click “My Account Preferences” in the left column and then click “Deactivate my account” at the bottom. Select the reason why you want to deactivate your account and click “Deactivate Account”. Your account will be disabled but you can reactivate it whenever you want, just by accessing your account.
URLS:
http://www.tuenti.com
http://www.tuenti.com/#m=Settings&func=view_delete_account_page