You should be striving to not allow spectators to inspect your props, UNLESS doing so actually works in your favor (for misdirection, etc.). Allowing the spectators to inspect interrupts the flow of your routine, and breaks down the invisible line between performer and spectator.
Allow spectators to see props on your own terms during the course of your trick, and move onto the next effect immediately.
I agree with everything this man has said. I only let the spectators touch my props, weather it be cards or what have you, when it helps me. Letting your spectators feel free to touch everything that you show will only lead to disaster, i.e- your example. Guide them along and give subtle indicatons to let them know when they can touch, i.e- at the end of my ACR the bent signed card pops up to the top. I dont take the card off this time to show them that it is their card but I remain silent just looking at the card. Then the specatator knows or at leat feels like he/she can take the card to make sure it is theirs. While they are in amazement, I put the rest of my cards away and move on.
Dont let the spectators lead you, you lead them.
If they grab at your stuff, slap them and say "I dont come to your house and pee in your pool, so dont come and watch my show and touch my stuff."
Hope this helps
(Sorry I dont have a spell check on this computer, sorry for any mistakes)