You can uninstall Timetastic from your G Suite account within Timetastic:
From here, head to
SETTINGS > INTEGRATIONS
Then, just click the delete icon to remove your G Suite integration:
Once you have removed the integration, your Timetastic account will disconnect from G Suite, and won't attempt to sync any more.
You can keep your Timetastic account and use it unconnected to G Suite, or you can delete it if you wish. To do that, simply head to Settings / Billing, and click "Cancel account".