This to me seemed to be the core of the problem; not that the cards were crackable, but that it was possible for a user to select what card to purchase rather than just what game to play. Remove that and the problem largely goes away through simple education about valid play procedures as I suggested.
(* I say educated, but there's the infamous 'Winter temperatures' scratchcard which rather undermines that concept - http://menmedia.co.uk/manchestereveningnews/news/s/1022757_c...)