You reduce escape risk by increasing affection and fear, and decreasing willpower. I recommend using the shrine to reduce will, but looking at your other questions it seems you may not wish to reduce all the NPC's will to zero? Spamming gold necklaces is an expensive way to spike affection, or repeatedly giving the day of is a longer cheaper version.
Thanks,
I might get a couple of necklaces to help speed up the process, but I've been lucky that a few of my newer slaves have "reformed" without much trouble (the ones that don't just go to the sawmill and mines where they are treated "nicely"). But I recently took over Silfren mine and want to take over Kahill mine soon, and I need to at least have some "happy" new guards. At the farm I am making more food than I need, so that's pretty much settled when it comes to personnel. Although I have been running out of bad guys to capture (except for those really hard.