How to get an iPhone app to forget all its data when deleting the app doesn't work?

I want to make an account in the hellotalk app, but it won't allow me to have more than one account per device. I don't remember my old password and no longer use the email address that I used to make that account, so there's no way I could recover the account.
I tried deleting and redownloading the app, no luck. It still remembers me. Anything else I could do?

Its probably checking your IP so nothing you can do.