Hi Miche, Here's how they sound to me, as a native speaker, but not a grammarian: 'I can't disagree more' --is fine. 'I can't disagree on that more' --carries an implied 'with you', as in: 'I can't disagree more [with you] on that'. ) 'I can't disagree with that more' --emphasizes the idea, action, etc.
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