Re: I DISBELIEVE YOUR REALITY! OR The couch is really there!
I think the "I Disbelieve!" response is an artifact of the original AD&D's mindset wherein, Gygax seemed to espouse an adversarial relationship betwixt GM and Players. But I'm just guessing.
Players who disbelieve everything they encounter is a symptom of a campaign where nothing can be trusted to be what it appears to be. If the campaign in question is one the players used to play in, a little experience with your campaign should eventually break them of Disbelieving Tourette's Syndrome™. Of course, if they started doing this in your campaign, you may have grown overfond of illusory attacks. Cut it out. The only reason illusion works so well is because most of reality isn't illusory (and all you philosophy majors be quiet, this isn't philosphy but gaming we're talkin' about! ).
You want to limit your players' ability to disbelieve? Give them clues when they really arefighting an illusion. That way you can simply tell them "Sorry, but none of the evidence I've presented to you is inconsistent with reality. Did you take 'from Missouri' as a psychological limitation?"
I think a reasonable way to simulate "active disbelief" would be to grant the person a breakout roll (with all appropriate penalties) with a +1 for each -1 DCV penalty the player takes versus the alleged illusion. Skill levels used for DCV should never be used in this case, to prevent players from just buying defensive skill levels for the sole purpose of disbelieving illusions.