A FireApps subscription gets canceled once you uninstall the app through Shopify or close down your e-store. You can remove an app from your Shopify store through your Shopify admin > Apps page. Locate the app you want to uninstall, and click the garbage can to delete the App.
Note: Unless you close down your store, Shopify will still charge you after 30 days installing paid plan, even you've already uninstalled the app.
If you change your mind later on, you can always resubscribe to FireApps. Just install the app, and all your data will be synced back.
Be noticed: Starting from Oct-2018, we will wipe your data after uninstalling for 3months. This will perform automatically so make sure to reinstall the app before that time.
If you find some code lines of the app still left in your theme CSS, please contact us for further support.