Thanz has a good point there are evil a**hats out there that need to be taken down at every turn. A hundred buck party trickster may not be the appropriate battlefield in the greater quest for truth, but if it really does upset you, then maybe you just play with the "fun" theme. Is there a company bulletin board? Right next to the party announcement (and the "readings" plug), hang up a notice of your own, completely sincere-sounding, that already points out how silly these things are.
"To All Staff
Hey, this is great! Everyone should sign up. We had a "fortune teller" at my cousin's bar mitzvah a few years ago. My uncle wanted to teach his son that becoming a man meant learning to think, so he had this lady come in who pretended she could see our futures in tea cups and a crystal ball. Man, she told people the most basic stuff, but said it so seriously they just gobbled it up! I still remember what she told me,... "Standing on your own has not been easy, but you are a person who likes to do things your way." Yeah, that kind of stuff! You can have a ball with this sort of thing at a party!"
There's no rule out there that you have to be a stuffed shirt pipe-smoking skeptic. Play with it and maybe you get them to play along, too. Don't compromise your true feelings or beliefs, but "go with the flow". Who knows, you might find two or three more skeptics in the office, and a good bit of fun for a group of skeptics is hanging around watching people get sucked into the silliness offered by a fortune teller.
(Umm, you might want to first sneak into the boss's office and see if he has autographed photos of Sylvia Browne on the wall. In which case, uh, forget it.)