This FAQ covers this: http://forums.adobe.com/docs/DOC-2971
or you can simply sign into this page using the same "AdobeID" (email you use to log into FormsCentral) and you can turn off your subscription here: https://www.adobe.com/account/my-products-services.html#MyServicesHeader
Once the trial was over you would not be charged an "annual" amount, the trial is for the month to month plan and you would be charge about $16 for one month (whatever the monthly rate plus taxes comes out to).
I thought the trial was only supposed to cost 99 cents, not $16. And I want to make sure that cancelling my subscription renewal doens't mean I have it for this year and it just won't renew in 2015.
The 99 cents is the first month, after one month (unless you cancel) you would be billed at the monthly rate for the "Basic" level plan at $14.99/mo plus tax. Cancelling the subscription means that after the one month it would revert to a Free plan, the trial is not for a year, it is for a month.
Below is the fine print from the pricing page regarding the trial (which at this moment is free and not 99 cents), this is all fairly clearly explained here:
The FormsCentral free 30-Day trial gives you the option to try the Basic plan features without being charged for the first month. This offer is for North American customers only. Simply add to cart and enter a credit card to set up your account. After you place your order you will have 30 days to use FormsCentral. At the end of the 30 day trial period your card will automatically be charged the standard subscription rate of $14.99/month USD, unless you cancel your subscription before the end of your trial period (subscriptions cancelled after standard billing has commenced may be eligible for a full or partial refund). To change or cancel your subscriptions, sign in to your My Adobe account and select the subscription you want to change or cancel. Under Subscription Info, turn the Auto-renewal setting to ‘Off’. You can continue to use the online service until the end of the billing cycle.