Thanks for your reply Aizaz.
When I try and restrict the categories, it only shows the categories with 1 or more products. The categories I wanted to exclude have zero products and they don’t show up in the dropdown field! I’ve just set the toggle button to on for Hide Empty Categories and it seems to have done the trick. Thanks for your help.
On a side note, if I turn on Show products count for each category, the icon is displayed really small. Is there a way that I can make the icon bigger using css?