I just tried to delete some non activated users and when I clicked to delete them the next screeen gave me this :

Admin user cannot be deleted. (User: 012972)
Admin user cannot be deleted. (User: camioneta1906)
Admin user cannot be deleted. (User: Cxtoon)
Admin user cannot be deleted. (User: ejackson)
Admin user cannot be deleted. (User: knudmals@hotmail.com)
Admin user cannot be deleted. (User: MarkJones)
Admin user cannot be deleted. (User: mastermine)
Admin user cannot be deleted. (User: rewini)
Admin user cannot be deleted. (User: Rorschach)
Admin user cannot be deleted. (User: runner11)
Admin user cannot be deleted. (User: Tibbly)
Admin user cannot be deleted. (User: wearingthearmour)