Wow, strange discussions. I tend to agree with despegue that the captain has the final decision (at least in my company), it's pretty logical as well and has nothing to do with the so called "Commanders authority".
There's 2 parties:
- the ground handling company
- the airline
My company gives me manager rights so that I can act in name of the company when I'm operating the aircraft.
The aircraft is property of the airline so it's pretty logical that the owner has the final saying on what is loaded, who is loaded, how it is loaded, what services are required etc...
If the ground handling company does not want to do that, that is their right. But they can not decide anything as it is not their property.
That being said, being disrespectful is never OK and very unprofessional.