Give your companions clubs, blunt tip lances or other blunt weapons as secondary weapons, not their primary ones, and when you want them to use them, issue the order "Only use blunt weapons." It works the same in PoP 3 as in Vanilla Native V 1.011. The first weapon you give them is the one they will use in melee if not ordered otherwise - if you are planning to try to take a lot of prisoners, you may wish to manually switch their blunt weapons to the first slot, but don't forget to change them back when you're done! Those members of your party not equipped with a blunt weapon will use their fists only, if you give the blunt weapon order.
Edit: If you are new to playing Mount & Blade, you may wish to do some reading about how the game works in general in the Apprentice Guild - commands and game options are explained there. We didn't put things which are standard to Vanilla Native in the FAQ, but some things in POP work differently from Native, so you may wish also to have a look at our FAQ.