you will probably have effectivly found a way to keep out those who honestly only wanted a card thereby taking away the only product you had in the first place.
I in the past had a struggle with this and put it on here on the forum for others to have a shot at answering this dilema. There have been several answers but none that I consider viable enough to make it work.
The jusry still out on this one ... but I have faith in the minds of man (and woman) that if we task the mind enough and expect an answer, the subconcious will deliver. Question is ... who's subconcious ... and when?
Lot's of creative minds here though and it will be soon I am sure.