Have a look at Geta.Optimizely.Categories repo they have an example here on how it can be achieved using an editor descriptor. I tested on a CMS12 site and looks to work 

geta-optimizely-categories/HideCategoryEditorDescriptor.cs at master · Geta/geta-optimizely-categories (github.com) 

   /// <summary>
   /// Hides the categories drop down list for types where it is not needed
   /// </summary>
    [EditorDescriptorRegistration(TargetType = typeof(CategoryList), EditorDescriptorBehavior = EditorDescriptorBehavior.Default)]
    public class HideCategoryEditorDescriptor : EditorDescriptor
    {
        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
        {
            base.ModifyMetadata(metadata, attributes);

            if (metadata.PropertyName == "icategorizable_category")
            {
                metadata.ShowForEdit = false;
            }
        }
    }

Thanks. I tried it in an Alloy solution here, and it works there. So I guess it must be something in our solution that is causing it not to work.