A customer was once getting arsey over paying a very petty bill, I called in their place to chase it up.
I ended up being threatened with the police, so I gave them the number for the front desk of the local nick.
Then they decided I was being threatening (there were four of them and one of me).
The guy then said "I cant afford it, what are you going to do about it?"
I said, "`phone the official receiver and inform him that you are trading whilst insolvent"
I got paid on the spot.
I know someone who dropped the bucket of a digger through (yes through) a portakabin office of a client that was refusing to pay.