This Entity Belong to Everyone (no it damn well doesn't!)

Well, I’ve been noticing lately that in single player mode and when I’m hosting, headcrabs can’t be killed with normal methods. I also recently downloaded the NPC spawner entity, and any NPC spawned with it can not be killed with firearms either. They only take damage from explosive weapons, like grenades or bombs. When I try shooting them, or remove-tool-ing them, a box appears saying “This entity belong to everyone”.
How can I get the entities to belong to me, and not the world? Preferably a method without having to shoot each entity with an own tool or something…

You don’t need an admin mod in single player. Remove it.