Knocking Creatures Unconscious:
See Lightning Bolt and targeting unconscious characters – Ryan pointed out a complaint that if unconscious PCs were valid targets then PCs would be within their rights to knock out an enemy instead of killing it and then use it as a punching bag for spells. My response would be that unconscious (though not dead) monsters would be valid targets – though they would not be “Legitimate Targets” (see below), unless there was a good reason to do so. This means you could do damage to the downed enemy, but would not get the effect bonuses for the hit. ie. you would do damage for the healing strike, but would not allow PCs to heal. Back to the unless there was a good reason, if you’d rendered a enemy unconscious to interrogate later, and then their healer got within range to raise him, then targeting the downed monster is a legitimate target and you’d get the bonus.
This whole hitting characters who are on the ground thing is an informal thing that I try to avoid because it’s not fun. I look the other way on area effects sometimes (and did in the last battle), and do not try to kill characters for fun. However, I think it should be clear that when necessary or advantageous for enemies to do so, it should clear what is allowed and expected.
Knocking Creatures Unconscious When you reduce a creature to 0 hit points or fewer, you can choose to knock it unconscious rather than kill it. Until it regains hit points, the creature is unconscious but not dying. Any healing makes the creature conscious. If the creature doesn’t receive any healing, it is restored to 1 hit point and becomes conscious after a short rest.
Legitimate Targets When a power has an effect that occurs upon hitting a target—or reducing a target to 0 hit points—the power functions only when the target in question is a meaningful threat. Characters can gain no benefit from carrying a sack of rats in hopes of healing their allies by hitting the rats.