If it is a card effect that requirres a set up with a normal deck of cards, carry two decks with you and do a deck switch while you are trying to 'talk your way out of it'.
Your best defense is one you are already using however, offer to show them something else. And have a lot of impromptu effects lined up to show everyone.
Or, if you get a moment to yourself, reset the set up. Or devise a method to do the reset quickly and unnoticed by your specs.
However I would NOT recommend showing the same effect again right away to the same person. Offer the friend an opportunity to see it later away from the first person who saw it and really built it up for them.
Just a few random thoughts on your problem.