LightRocket will automatically debit your credit card when your account is due for renewal. This is to avoid your account expiring and your files being deleted because you forgot to renew.
However, it's easy to turn off automatic renewal if you prefer.
Log in and browse to your dashboard and then to the 'subscription' page or alternatively browse directly to the subscription page from the drop down that appears under your avatar.
You'll see a screen like this...just click the 'cancel renewal' button and you're done.