If any of the girls on the hen night are likely to get up to any naughty behaviour and you feel the need to bring them into discipline then why not threaten to lock them into a pair of handcuffs?
Luckily these do have a safety release system so whoever is wearing these can get out at any time, although if you feel like pretending that you have lost the keys, then you have a classic hen party prank in the pipeline.
These are made of plastic, but look extremely realistic, so any bystanders who happen to see them will never be able to tell the difference.