When no DM is present, PCs cannot emote subduing (knocking out) spawns (bandits, monsters, animals, etc.), unless they actually have and use the means to subdue such spawns (for instance with a sleep spell).
If a DM is running, then subdual may be possible; ask him how he wants to handle it. If he says no, accept the ruling without argument.
Rules regarding regular NPCs have already been posted, but to reiterate, they cannot be attacked (including subdual) without DM permission.
Edit: Yikes! I confused a player! Here's more detail:
Hostile spawns are of course also technically NPCs, and you can still kill them without DM permission just as always. What you cannot do (and what this post is about) is kill a spawn and then emote that you knocked it out.
By regular NPCs I mean non-hostile merchants and townspeople and the like. You can't walk up to Gretta and cut her down without DM permission. You can still, however, hunt non-hostile animals (such as deer) without DM permission.
13thHour : [Tell] *your alignment has long since passed any possible further move to 'sexy' due to reinventing the scale*
Lexy on the Wiki!