By default, the Instant Search Widget shows categories/collections that match the search term. 

To hide categories/collections from the widget, follow these steps:

  1. Go to the Smart search & Filter control panel > Instant search widget section > Content tab > Pro features part.
  2. Set up 0 in the Categories qty. field. 
  3. Apply changes.

That's all. The categories/collections section won't be shown in the widget. You can go to your storefront and check it out.