To close your account and thereby delete all your data, the balance on all your lists must be 0. If your balance is not 0 on a list, you can add a transaction that will put you at 0.
In the app go to Account > My Profile. Here you will find a link to close your account.
On the website follow this link: http://app.splitser.com/account/delete
Note: We cannot close accounts via email requests. For security reasons, you must be logged in and perform this action yourself.