C++: Remove == or any other operator for specific types

If I have a class called classA that has a typecast overload of classB and classC, how can I disable the == operator for classA being typed as classC?

class classA{
   operator classB();
   operator classC();

   //Remove this comparison possibility:
   operator ==(classC firstObject,classC secondObject)
}

Follow over from this question:

Ambiguous overload when using many typecasts operator overloads