When he interrupted me while I was demonstrating I cut him off quickly and said "We can discuss it after class or at open mat" or I specifically explained to the class in detail why what the guy was saying wouldn't work for what we were doing.
I was respectful and firm.
Whoever should be teaching in front of the class shouldn't be interrupted or challenged. This is part of the etiquette in an aikido dojo. Even some dojos are very strict in talking during practice. talking during practice is acceptable but interrupting whoever is in front of class teaching is unacceptable behaviour.