As Much Use As A Cock in a Nunnery

Some people are useful. Some people are really not. These people are as much use as a stray penis in a sacred nunnery where all potential partakers of the penis have actually given the solemn vow of chastity. Therefore the penis is really of no use. Hence the simile to suggest that someone is really useless.

That guy Carl is as much use as a cock in a nunnery. He is absolutely useless