That was actually my first idea, but everyone knows that the odds are with the house, and as people pointed out the odds are printed right on lottery tickets and people still buy them despite the terrible deal on offer.

I figured that adjusting say 8% of the ads to reflect an (imaginary) 8% incidence of problem gambling would actually be more effective, by balancing the legitimate commercial desires of the casino, the legitimate worry of gambling addiction, and making the billboards just unpredictable enough to keep people playing...er, guessing.