Even before entering Helium, be careful, there are lots of complaints about the site, especially because they will not delete the articles written by you once. Actually your texts become their property. By the way, you agree to this when creating an account, so it is your responsibility from the first moment!
How to delete your account: from the email address linked to your Helium account, send a mail to email@example.com. The subject should be “Cancel my Helium account”. In the mail, request account deletion.
For the interested, some drama experience by users: http://tamilynnk.tripod.com/JustSayNO/index.blog/1816216/contents-of-emails-from-helium/