Click on "Edit" in the top left-hand corner of the QuickBooks window.

Click on "Preferences".

Scroll down and click on "Tax: 1099".

Click on the "Company Preferences" tab.

Click on the blue font that says "you can do it here" in the second option that says "If you want to map you accounts to boxes on Form 1099-NEC,"

Click "Yes".

Look for the account number/name you are trying to unmap. Once you locate it, change the selection in the third column to the right of the account number/name to "Omit these payments from 1099". 

Click on "Save & Close".

You're done. You should now be able to make the change.