You can find an SCSS solution as well as a Javascript solution in this forum post: https://forum.mendix.com/link/questions/85644
Hi Pedro,
1)If you are using Reference selector then use X-Path Constraint [Name != ‘(vazio)’]
2)If that is ENUM then use DynamicEnumPicker Widget to hide option. https://marketplace.mendix.com/link/component/111237