Unfortunately we could not find an automated way for cancelling your account. You’d better send them a mail to ask for deletion (use the link). This goes for all Groupon sites (e.g. groupon.de, groupon.nl, etc.).
This is stated in their Privacy Statement (nr 5). http://www.groupon.com/privacy
Nr 3 in the Statement is about sharing your personal data (which in some cases they will share for commercial purposes), so read well.