I had the same issue. I think Turbotax seems to be very tricky on this. It made the deletion of the right forms very inconvenient, and I was forced to upgrade last year, although I needed not to.
This year I figured it out. It was mainly because I accidentally click "Less Common Income" and peeked into one of the options. There it was prompted that I must upgrade in order to fill that form. I said "don't upgrade". I thought I did nothing there so it should be fine. In fact, Turbotax already created an empty form for you. This is definitely an unwanted feature! After I completed other forms and tried to file my tax return, the only option is to upgrade because I have "miscellaneous taxable income". Note that such a name cannot be found when you click the less common income. So it is difficult to figure out which form is the devil.
What I did then is click "Tax Tool" on the main list, and then select "Delete form". There was no form which had 1099 or Miscellaneous Income as part of its name. This added to the difficulty of finding the right form to delete. I think I tried "other income worksheet/form", then the mandatory upgrade disappeared. Turbotax could have made this process much easier.