There were two kinds of boundaries put in to stop the practice of leading NPCs away from an area so that a player could do the pents faster, without interference.
1. Physical blocks to stop the NPCs crossing a boundary. There are a few of these implemented but it does cause a few issues with clumping on the boundary edge. Hence this solution was not extended beyond the initial problem areas.
2. When the pents are solved the pents check to see if the NPC is more than X away from the home pent that spawned it. If it is then the NPC does and is respawn back at home. This applies to all pents and all NPCs in the pents. I do not recall what X was but would guess about 15 squares. The name of the player who led the NPC to their death is recorded in the logs.