Uninstalling Fomo on Shopify is a 2 step process. After logging into your store, you should go to the Apps menu. Inside the "Installed apps" section, search for Fomo.
Click the red "Delete" link on the side of Fomo.
This will remove Fomo from your store and it will also stop any future Fomo invoices.
Note: cancellations are permanent and you will lose your Fomo setup (any Settings, Actions, Rules, etc.) when you uninstall.