The stock market is heavily regulated. I don't think you should ban sports betting, because like many vices it's easier to control if it is legal. According to Nate Silver, the more you let people make bets on obscure things the more opportunity their is for participants to cheat. So you should probably restrict betting to things no one participant can control (like the score). You also should try to make it difficult for a person to lose too much. You can't stop it, but you could probably make it harder. In the stock market there is a "qualified investor" that is allowed to take much bigger risks. You could make rules to punish betting sites that accept too many bets from destitute addicts. It wouldn't be perfect, but you can have liquor laws without having prohibition.