Reminds me of an incident once at my job. I had a co worker who claimed he'd been in prison for manslaughter. Any rate he got angry one night at me and started to threaten. Said he'd be meeting me out back after work. I just smiled at him and said ok and continued working. He kept at it and I just kept smiling and saying ok. Totally relaxed and quite cool.
About 30 minutes before we were due to be done work he came over and apologized to me and said he was just upset and didn't mean it... all the while his body language said he was not quite so sure he wanted to go through with it.
Its not the first, or last, time I've used the same body language mentioned by Kevin to change the mind of someone who thought they could push me around. One thing I've often said. You don't actually have to be able to kick someones but you just have to make them believe you can.