You could also forcibly capture them, such as disarming them, so they have nothing left to fight with (assuming they have no magic left or they cannot do magic).  Or you could see about restraining them (which would be the case for "taming" wild monsters without magic).

  Morale does not and should not have everything to do with capturing enemies, since if they get disarmed (so there is nothing that they can do to fight back), they would probably surrender or try and run, or in the case of bring restrained (either by some sort of magic restraints or by ropes/chains), they could still have high morale, but have nothing they can do except be captured (except if their side wins or with magic, they could possibly use it to free them selves or teleport away, again assuming magic is not being used to trap them).

What would be the mechanic for it though?  I mean lets face it, very few single combats would ever be decided because someone had been "disarmed."  Multiple weapons combined with that fact that hacking at each other with as sharp (or heavy and blunt) an object as possible does not lend itself to draws means that a single fight probably isn't going to end until one person is down.  In a bigger combat odds are other people are fighting around you, and unless one side is breaking and running you don't have time for prisoners because there would be lots of other threats out there (conversly, there would be lots of dead for that disarmed person to pick weapons off of were you to turn your back).  This is why very few prisoners were taken in medieval warfare until after an army had routed (which was usually pretty early... they weren't the most disciplined bunch).  I just don't see the real need for anything other than the morale of an individual or unit to determine surrender conditions.