You can stop your subscription by clicking "cancel subscription". This will stop you from being billed again. You can do this on your account page: https://my.sonix.ai/account.
Note that when you cancel your subscription you lose access to your files.
You can however, purchase a storage plan to maintain access. The storage plan is $5.00 per month per 50GB. If, at some point in the future you want to re-subscribe to Sonix, you can easily do that on your account page: https://my.sonix.ai/account