You can't. This is because of IRS and other rules that have to be followed in order for TurboTax to provide online services. What you can do, is delete any tax return you may have already started on that account. (I have a 2nd account too, which I will use for estimating and the such, so I don't mess up the real one.)
In that account you don't want, open the return (if you started one) and then click My Account and select the clear and start over option. Work through a few screens to complete the process, then log out.