Quote Originally posted by Uzi:
.. but how do you do it?(technically)[/QUOTE]
Drag and drop his unit (in the lower centre of the strat map) onto another unit of the same type (eg royal knights) in the same army, which will transfer knights from his unit to the other. Once his unit strength is 1, right click (again in the lower centre of the strat map) and select "Disband" in the lower centre of the parchment that appears.