If you don't want application anymore you can simply remove it from your Shopify admin.
1. Log in to your Shopify admin.
2. Go to the "Apps" section
3. Delete the application
4. Feel free to give us some feedback
If you follow these steps you should no longer have the application and Shopify will not charge you for any further use.
If they do charge for the application after removal, you have to contact Shopify and let them know that you want to get a refund. Shopify will then contact us to confirm the refund.