Unfortunately, once you have started preparing your return with a spouse, there is no way to remove them. You would need to create a new return to prepare a return only for yourself. 

To start a new return, select “Start a New Return” from under “My Returns” in the bottom left corner. Once you have started your new return, you can select “View All Returns” from under “My Returns”. Then delete the unwanted return by clicking the small Trash Can next to it or “Delete” under the return’s name.