Hi there, 123joan321.

Thanks for dropping by here in QuickBooks Community. Allow me to help and address your concern, so let me get this straightened out. 

Currently, there's isn't an option to turn off the specific auto fill for the account field in QuickBooks Desktop Mac. I can see how this future would be helpful for you, I recommend sending feedback directly to our product developers for consideration in the future updates. 

Here's how you can send feedback: 

1. From the QuickBooks menu, click QuickBooks Feedback.
2. Select Product Suggestions in Type of feedback section.
3. Fill out the necessary information and click Send Feedback. 

For future reference, you can always check this article: QuickBooks Desktop for Mac 2019 User’s Guide.

You're always welcome to post if you have any other questions. We're always here to help you out. Have a good one. 

You can turn off autofill though for all fields. 

Also if you tab to the field it will be selected and when you type it will be replaced.