I doubt you can ruin gameness but you can get the dog to a point he never gets a chance to show it. Like a chain accident early on or being left down to long they never get a chance to enjoy what they are doing so they never will show you how far they will go. If that makes sense. Game is such an elusive term and has so many definitions depending on who is doing the defining. A game dog can be physically stopped and some will say cur others can see deeper into the subject. A winning dog can be a cur but never ran across anything to show him his character flaw.
I do not think one can ruin the genetic pre-disposition to be game but I think one could ruin a dog before game becomes a question, or better said, one can ruin a dog before gameness should be in question.