You can kill them and reanimate them as Zombies (You Need A Grave Yard) or get them low on health and use Shackles that you can get from Item Market. (You Need A Trading Post) I hope this helps.
You can also craft the shackles yourself using cloth. Scrap clothing items in order to gain the cloth, it takes 6 scraps to craft shackles.
Some of the races also have the ability 'Capture' which can be used during combat.
I know Imp has it, and I think Hobgoblin as well and at least one other race. Can be checked upon character creation.
