Unfortunately, even with teleportation cut off on the plane, the assassins are incredibly elusive. They seem to be capable of teleportation similar to that of the Shadowdancer, allowing them to escape, despite the planar lockdown. It takes the better part of two hours to track them down, and that's only because they're finally dead, their bodies reduced to a fine, dusty powder, leaving only a small, twisted stone ring lying in the pile, burned and badly damaged, but still intact.