How do I stop TurboTax from proceeding with our 2020 taxes?

You can just abandon your return. We can't touch your return or delete it.

TurboTax is not going to "proceed" with your tax return.   Only you can file your own return.   If you do not want to go forward and file it you do not have to .   If you have not registered, paid or e-filed yet then you can clear out the data that you have entered.

https://ttlc.intuit.com/questions/2586254-how-do-i-clear-and-start-over