He won't lose any money in slippage if he puts up a limit order (it might sound imbecile to some, but most people do place market orders, including the ones that think it's imbecile).
People rarely change defaults... how about as a first step making the default a pre-filled limit order routed only to Bitfinex? It would be nice to set the "only to Bitfinex" default to something else in the account settings of course. Not claiming a BTC for that one, but if you think people just quickly clicking buttons are a problem, do something about it!