just say "please participate properly in this trick or im afraid ill have to stop". if the audience are enjoying the trick they will moan at the guy who will then get embaressed and continue or if the audience moan at you for saying you'll leave just say "don't moan at me he's the guy who spoiled it"
(point to the guy who is ruining you're trick) then the audience will moan at him
hope this helps!
Not sure that's the best idea, shaming or putting spectators in an awkward position is never a good thing. Also making light of the situation is probably better than "please participate properly or i'll stop", 'cause if they call your bluff on that one you're going to look a tad foolish. I personally would just say something along the lines of the good old fashioned "OK, let your mind go completely blank... that was quick" but obviously slightly modified for the situation. If you actually know his card is black, then maybe say "I just did, I only wanted confirmation" as it dimisses his smart-alec comment in a polite way
But really I guess it's up to you if you want to take the spectator seriously or joke along with him/her