To delete your account, make sure you are logged in and then choose the Your Settings option from the user dropdown in the upper right. Then click onto the Profile tab. In the lower right of that area there is a red link to Delete Account.
This is a completely destructive action which deletes all Pens you've made, comments you've left, data about Pens you've hearted, and everything else you've ever done on CodePen. If you want to save Pens, we'd suggest saving them to Gist (which you can do from the Share dropdown on any Pen) before deleting your account. If you are sure, click the appropriate button from the dialog box that comes up.
Again a reminder to be sure before proceeding. We can't help you recover your account after deletion.