This can be done in just a few clicks, @Kaprow.

I got the steps that you need in disabling your closed bank account in QuickBooks Self-Employed.

You'll just need to toggle off your bank account in the system. This stops QuickBooks from downloading new transactions, and you keep all the existing data.

To do that:

1. Open QuickBooks Self-Employed in a web browser.
2. Select the profile ⚙ icon and then Bank Accounts. 
3. Find the account you want to hide.
4. In the Show Account section, select the toggle to OFF. 

To learn more about this process, here's a link that you can check out: Want to delete your online bank or credit card data?

In case, you want to import new transactions, you can always use the WebConnect process. This way, you can upload the entries in QBSE manually. 

I'll be right here to help if you need anything else. Have a great day, @Kaprow.