Ted here from Shopify, thanks for reaching out to us. :-)
It's not possible to pause a plan for an app. You would instead need to delete the app fully to stop the monthly charge. You can always reinstall it no problem further down the line if you would like to use it again. If you would like to delete the app, you will just need to click on Apps in the main menu, and then the garbage icon next to the app as shown here for example.
Feel free to reach out to our Support team any time as well. We're here 24/7 via Facebook, Twitter, chats, emails and phone. Just click 'talk to support’ for some more info.
How can we know what is removed when deleting an app. For instance, I want to remove an app that I used to build an FAQ page, should I expect the FAQ page to disappear? or will it still be there, but potentially broken because, for instance, styling is removed with the app? or will my page remain the same even after I remove the app?