Did you install Fomo through the Ecwid marketplace? If you want to uninstall Fomo and stop Fomo-related billing, then you will need to uninstall from your Ecwid account.
Within your Ecwid account, navigate to My Profile > Billing and Plans:
Click "Manage apps":
Click on the row with Fomo:
This will open up extra options that look like this. Click on "Uninstall" in the bottom right:
That's all! You've now uninstalled Fomo and will stop being billed from Ecwid for the use of Fomo.