Um, or more tactful argumentation. Try this: "This is an excellent idea! Let me try to add something else to it..." and slowly lead on to an idea that is actually correct and let them slowly buy into it, and sooner or later they will figure out on their own that then the original one cannot be correct.
I can't remember how I found this, just that I was amazed at how rational and near-mode it is on a topic where most of the information one usually encounters is hopelessly far.
LessWrong wiki link on the same topic: http://wiki.lesswrong.com/wiki/Status
Source: http://greenlightwiki.com/improv/Status
Retrieved 20 March 2012