Most SB business is on about six or ten main indices and FX crosses. The more punters they have, the less skewed their net position will tend to be and the less they'll have to hedge.
The tricky bit is when someone tries to put a large stake on something, particularly on an illiquid market. The majority of bets are for low stakes (I think a couple of quid on the Dow, maybe £5 on DAX/FTSE) so these can usually be filled automatically, letting the punter get on with losing. Anyone who demonstrates an ability to make a profit consistently will be screened and put on dealer referral, although the SB cos will deny this, of course. Those fond of conspiracy theories will also claim that all clients' betting patterns will be analysed so they know which ones never need to be hedged!