You would have to saturate the market with specific mining devices. Like with bitcoin ASIC miners, the crypto mining demands have to increase to the point where using gaming graphic cards becomes unprofitable. Otherwise miners will buy the specific mining devices and the gaming devices. The problem is that Etherum was designed to resist ASIC approaches.

Instead of locking cards down, Nvidia could manage the supply. They could sell their cards in cooperation with Steam: Only people with an active steam account are eligible to a new card. If their performance stats don't improve after the purchase, which means they have resold their hardware, they won't be allowed to buy another card.