If you have a paid account, monthly account holders must first cancel the auto-renewal by going to "User Menu->Billing" and clicking the Cancel Service button. Yearly account holders can optionally enable auto-renewal on the "User Menu->Billing" page and clicking the Summary tab. Deselecting the "Auto renew" checkbox will disable the auto-renewal and let the annual membership expire.
Trial customers will automatically revert to the Free service level when the trial period is complete.
Once your account is at the Free service level you can delete all of your forms. Free level accounts will automatically be deleted after an extended period of inactivity (over 1 year).