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 all your transcription data will still be in Sonix after you cancel. You'll also be able to upload and transcribe new files at the $5 per hour rate up until the anniversary of your subscription. After that you won't be able to upload new files.
If you want to transcribe with Sonix again at some point, you simply re-subscribe or purchase more minutes on pay-as-you-go the Standard Plan.