Unfortunately, in TurboTax, there is no way to remove a spouse/partner once you have started to prepare your returns together. You would need to create a new return and choose to prepare the return without your spouse/partner.
To start a new return, select “Start a New Return” from under “My Returns” in the top left corner. Once you have started your new return, you can select “View All Returns” from under “My Accounts & Returns”. Then delete the unwanted return by clicking the small Trash Can next to it or “Delete” under the return’s name.