I looked into this a little bit further, and actually you don't have to do so much just to uninstall the unneeded states; the state modules have their own standalone Windows installers! (which includes built-in support for the uninstall operation)
What happened to me is that I had briefly selected NM instead of NY somewhere on the return by accident. Once TurboTax saw that, it flagged the NM state module in the Windows registry to be later downloaded and installed. The flag apparently doesn't get cleared even if you later remove the reference to the state before the install actually happens. So I never had any need for NM and really didn't want to see it hanging around on my computer forever ...
It turns out, at least as of TurboTax 2019 desktop, that the installers for the state modules are downloaded and stored under the hidden directory tree at C:\ProgramData\Intuit\Common\Update Service\v4\Data . They have names like, for example, wctiper.msi for CT Personal Income tax or wnmiper.msi for NM Personal Income Tax. Once you find the installer for the state module that you want to delete, you can easily invoke the uninstall operation using "Run Command" or in "Command Prompt" like this:
MsiExec /x "\path\to\w??iper.msi" .
That will prompt you to confirm that you want to uninstall the state module, then in a second it'll be gone. You can also then delete the installer itself and the subdirectory it's in if you want as well (may not be necessary though).
Now, there also is the issue that TurboTax will re-download and re-install the removed state if it remains flagged by TurboTax in the Windows registry. TurboTax seems to maintain a separate set of these flag on a per tax year basis, which means that the flagging shouldn't be carrying over to subsequent years if you no longer needed them on the new return. However, you can check and delete them if necessary out of the registry with REGEDIT.EXE.
For TurboTax 2019, here:
HKEY_CURRENT_USER\Software\Intuit\TY19\PREFILLPERSTATES
I assume that for TurboTax 2020 those state flags will be in TY20 directory path instead.
Under each key you'll find a separate name-value pair for each state, just delete the one relating to the state that you don't want re-installed. As long as there is no reference to that state in your return, the flag won't be re-written into the registry and the state module will stay uninstalled!
