just alert everyone that they will be engaging in a paintball event; and they consent to the assault.
(I suspect the main objections to indoor paintball will come from the venue. Perhaps a less messy, but still humiliating, method can be used?)
That said, that's the least important reason, the more important one being that you can't just nail people with any kind of gun at an infosec conference, no matter how unconf it is.