As fun as it is to prove these people wrong again and again, it might be cool to fake them out too. People have done this before too, leaving sets of empty clothes around the office, sitting on the chairs to make a christian co-worker think that people had been "raptured" out of the office and that he or she had been left behind. It might be fun to wander around with halloween makeup in zombie hordes, or to dress up in devil costumes, and pretend to writhe in agony in front of churches. The zombie hordes would be a reference to the fact that even the long dead are supposed to be resurrected. The devil stuff would be just for fun.
It's conceivable that a person could use a powerful LCD projector and project an imagine that looks like Jesus onto a cloud, perhaps from a high building. I wonder how many people would be taken in by that kind of thing. At least it would be based upon some kind of evidence, unlike the stuff from Camping, et al.