We recommend you keep using online backup service to make sure that you don’t lose any notes. However, if you have to stop it for some reason, you can choose to disconnect online sync. If you disconnect online sync service, sync will no longer work. And your current notes will remain on your device.
[Settings -> Online sync service -> Menu -> Disconnect]
Note that once you disconnect online sync, your notes will no longer be backed up to cloud storage. Therefore, make sure that you back up notes to your device.