If you need to turn off your sync in Formstack Sync, all you will need to do is simply go the dashboard page and click the off sync status button.
Then click the stop button in the pop-up window.
Once you click stop, this means that Formstack Sync won't start any new sync runs for you. However, your sync may be currently running, in which case it will complete its current run before stopping completely (this may include creating and updating records).
To check if your sync is running, see How to Determine if Your Sync is Running