Step 1: From the back-end of your WordPress application navigate to Products > products
Step 2: Hover with the mouse over the product on which you want to disable the quantity field and click on Edit
Step 3: On the newly loaded page scroll down to Inventory and click on it, after that select a tick on Sold Individually.
Step 4: When you are done with the changes on the product, simply apply them by clicking on the Update button.
Step 5:This way you can easily disable the quantity field for a certain product.