Yeah, I think you nailed it. A ban on playing against "the house" would do it. Taking a fixed amount from each pot (as at poker tables) for play among patrons would still be allowed, but slot machines wouldn't. Your solution's much better than a full ban because it wouldn't drive as much illegal betting (a problem no only because it circumvents the law, but because for gambling in particular but for any black market, really, it tends to become connected with other criminal activity)