Hmm well most people here seam to have a flair for the dramatic. There is a lot of advice going around in the "stop the trick/scold/embarrass categories."
General solution
I would say you should perform a non-gimmicked trick first.
In my opinion this solution is the closest to what I employ. I find that if you give the specs an opportunity early on to snoop around, and control when they are permitted to do this, you can in a sence get the snooping out of their system.
I do not mean something corny and formal like "and please shuffle through the deck and make sure all cards are normal..." that would arouse suspicion. But in truth all the people watching are wanting to investigate.
All I do is wait until they ask to snoop and allow them to, this gives off an air of control. It fills them with the idea that they can check up on things any time, but the result is that they no longer feel the need to.
As the tricks progress you can get away with more and more.
Last resort
Hmmm if some one is on the brink of spoiling your trick,
DONT LET THEM. In one of blains videos he is doing ambitious card, and you can see a spec reach for the top card.
Now, had the spec turned it over, he and Dave would have been very disappointed. But Dave did what he could. He says very sternly, "Wait," and then pushes the man's hand away, and then very plainly says, "Here is the move," and executed rist flick, and reveals the signed card with the DL. Disaster averted.
It's all about damage control.