Sometimes a product has only one variant. In some stores all products may be single variant products, and the variant title is left as ‘Default’.

For these products you don’t need the dropdown selector in the notification form.

You can now hide it with some simple Custom CSS. Under Widget appearance, add the following rule to the Custom CSS field:

select.single_variant_product { display: none; }

Refresh the product page and you’ll see the dropdown is hidden.