I do a trick where they take a card, return it, and at that point I do a double undercut to bring it up top.
It works fine mostly, except for a group at school that insists pushing the card way into the deck and squaring it up. Thus I don't get the chance to put a break in, and thus no trick.
I've tried the suggestion of holding the deck tightly so they CANNOT push the card all the way in, but they almost take the deck from my hands to do it

Any ideas?
