There is no way to delete a return filed through NETFILE without deleting your account. If you delete your account all your returns will be permanently deleted and you will not be able to retrieve them or carry forward your information to next year.
TurboTax Online uses the same security systems that banks use. Your data is encrypted both while on the server, and when the return is transmitted to the CRA.
If you would prefer not to have your return data stored on the TurboTax servers, consider using TurboTax CD/Download (for Windows), which only saves returns to your computer.