Dread Collectors are a special enemy that will only appear if you are caught stealing from a shop. They are fairly weak in health, so if you have a magic Katana, they will be easy to kill. However, they have a lot of attack, so unless you have a lot of advanced armor, such as the Imperial Boilerplate, they will be very hard to deal with. To stop them from spawning on a floor, you have to kill every Brax on that floor, not just the one you originally stole from. This will not remove the Dread Collectors that have already spawned (and there will probably be plenty,) but will stop any more from spawning.