All App.net paid accounts are defaulted to auto-renew at the end of their billing cycles. If you would like to prevent your account from auto-renewing you will need to "Downgrade" your account to the free tier.
The downgrade process will immediately transition your account to a free tier account.
Click Here for more information about what happens when you downgrade/fail to renew your App.net account.
Click Here for information on how to downgrade your App.net account.