First they have to be held in a prisoner cage in your own outpost, then threaten to sell them, say they aren’t worth hauling, and tell them they are only valuable for shoveling ♥♥♥♥. I think it’s about a 50% chance if you do all these.

Also don’t pay them, somehow they can still steal from you in a cage.

I think this the same for Big Al their leader too (but I didn’t try because his stats). Also this should apply to any other Swamper type.