You cannot remove organic limbs, even if they have a scar on them. installing and removing peg legs / steel arms should do the trick for taking their limbs off. Alternatively, if a pawn gets an infection in a limb there is an option to amputate. Easiest way to do this is use a shiv to cut them up a bit then give them a sleeping spot on a dirt floor. be sure to disable medical care so your doctors don't seal their cuts. Be sure to save beforehand in case you gets scars from this.