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 re-subscribe to FireApps apps. 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 3 months. This will perform automatically, so make sure to re-install 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.