Been a long time since I programmed thief stop. Originally, it would freeze the thief, but people found a workaround. If I remember correctly, they started working in teams. As soon as one was frozen/caught, the other would unfreeze them or do something to rescue them. That's why thief stop now send the thief directly to prison.
As for stolen items, I believe they are supposed to be returned, but it isn't just the ones stolen from that particular person. It's anything still in the thief's inventory that is marked stolen. This was tested a long time ago, but I don't know if it's still working.
I thought about adding some other types of thief stop to liven things up. Maybe ones that would cause the thief to explode or melt or whatever.
Just to clarify, thief stop only works if the thief steals it from your inventory. Let's say you have 10 thief stops and 10 other items. That gives the thief a 50/50 chance each time of getting caught.
Other things you should know:
- You can be pick pocketed when you are dead, so don't lay around in dungeons.
- True sight only let's you see the thief coming. It doesn't stop pick pocket.