Challenge accepted.

Ricky Hatton's (pro boxer) punch speed - ~25mph
Diameter of a H2O molecule (most common molecule in human skin) - ~0.29nm (0.000000029mm)
25mph = 11176000000nm per second
Deceleration = (0 - 11176000000nm per second)/0.00000000025948461 seconds = -4306999200000000000000000000nm per second per second.
-4306999200000000000000000000nm per second per second = -430699917039m per second per second, or -430699917.039km per second per second.
(If you don't want to read that brick of a number, that's over 430 million kilometres.)
-430699917039m*0.7 (average mass of a human fist) = 301489941927N
(Or three-hundred thousand million Newtons of force.)

So how much is ~300,000,000,000 Newtons of force? Well...[list]
[*]About 3300 Newtons will break a rib.
[*]About 250,000 Newtons will instantly stop a car moving at 60mph (depending on the car).
[*]About 750,000 Newtons are necessary for a rocket to lift off.
[*]A 50-kiloton nuke delivers about 450 million newtons of force.
[*]And since I'm running out of ideas, it would take about a 34 megaton nuke directly hitting this person's fist to re-create the damage that one punch would cause to them.[/list]