You can uninstall Timetastic from your G Suite account using the G Suite admin console (https://admin.google.com).
From here, head to
APPS > MARKETPLACE APPS
You'll find Timetastic in the list of apps, simply click and then choose the delete icon to remove Timetastic.
Once you have removed Timetastic from your G Suite account, 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".