Ask the person you are debating with if they are debating what's canon, or what their interpretation is. Remind them that you can't pick and choose your canon, the way so many people tend to do. Canon is a seperate entity from your own interpretation.
The Ultimanias are meant to settle what is canon, or in another term, what the truth of the story is meant to be.
Sometimes if people were being really stupid about it I'd give them a history lesson on what the word's original meaning was. (Hint: has to do with the books of the Bible).
If they want to ignore the Ultimanias, then they can't be debating canon. If they want to discuss personal interpretation, that's a whole other topic.