Our friends from 708 had some helpful advice.
- Put sticky tape on your ass... so their hand gets stuck.
- Springs. Large metal springs so their hand's slapping force projects back onto to them (think Newton's law of force).
- Tacks
- Frog Poison - dead on contact!
- Magnesium phosphate - this will cause the culprit to explode upon force, although you may suffer some risk...
- Knives