If you don't want anyone in your organization to use Mailmeteor, you can remove it from your domain. When you remove an app, none of your users can access it.
Go to your Google Admin console (at admin.google.com)
Go to Apps > Marketplace apps.
Click on Mailmeteor.
At the top of the app Settings page, click on the bin icon
Click Remove App. Mailmeteor and any associated licenses are now permanently removed from your domain.
Go to the G Suite Marketplace
Sign in with your Google account.
In the left sidebar, click on Manage Apps with the gear icon
You'll see a list of administrator-installed apps and any apps you’ve installed yourself.
Locate Mailmeteor and click Manage > Remove. Mailmeteor is now uninstalled from your account.
If you want to cancel your subscription, you can follow the steps described here.