I've used a standard question to throw voodoo spooks when they start babbling excessively for many years now (so long I can't cite the original idea) -
Why aren't there any psychics barred from casinos ?
Surely, if casinos can instantly spot and eject card counters, it shouldn't be too hard to deal with psychics ?
More recently, I've come up with another one -
Why don't professional dowsers volunteer their services to clear third world landmine fields ? Not enough faith ? Or do they simply not care for humanitarian work ?
Anybody have any other ideas ? This would be useful for atheist-fu when dealing with nutjobs.