TurboTax automatically entering last year's unallowed rental loss as this year's net operating loss?

I think you mean you are using TurboTax Home & Business.

TO DELETE THE NOL

Select the Business tax and scroll down to 

Less Common Business Situations

Scroll down in the section to 

Net Operating Loss/QBI carryforward Loss    START or UPDATE

Clear out any numbers, replace with 0 

OR

For Desktop, Switch to FORMS (top right)  

Scroll down to    Carryover Wks    and delete 

TO ENTER THE PASSIVE LOSS CARRIED OVER

Go to the Rental Section

If the rental is still active, select EDIT for that rental 

Select "Property Profile"    

Third screen in asks about Carryovers, Select "I have passive activity real estate losses carried over from a prior year" and CONTINUE

Enter the amount on the following screen. If you have not been subject to AMT, enter the same in both boxes or adjust for the AMT amount if applicable

If the rental is no longer active,  

Go through the rental section to get to that same screen. 

Enter the property the loss was taken on for the profile to get to that screen 

Select only the answer to enter the loss carryover

Select NO it was not rented all year  Enter zero days rented zero days used for personal use

DO NOT ENTER INCOME NOR EXPENSES

The loss will be listed on Schedule 1 line 5