This is sadly another imported menace that costs us money and manpower to deal with. I hate the idea that there are any people forced into marriage around the globe but we may as well help the ones going on under our noses.
First of all, zero tolerance. If an under 18 year old female (or male) goes missing from the system, the authorities should dig deep until they get to speak to her alone, in person. If the parents drag their heels, are unhelpful or simply refuse to reveal her whereabouts, arrest them for suspected murder and give them no bail. They'll soon come to their senses if old Ma Khan is stuck in a remand cell with dykes and dirty infidels.
The thing to remember is that under 18's are children, they are vulnerable and they have every right to a happy and safe existence. If the parents can't accept the concept of freedom in our society, they must be robbed of theirs. It will also act as a powerful deterent to any other "loving" parent thinking of in essence, pimping their offspring out.