Are there any idioms to describe someone as being unimportant and to say that he is useless?
My favourite comes from Casablanca
'You despise me, don't you Rick?'
'I guess I would if I gave it any thought.'
They don't write dialogue that that any more
I've heard lots of variations of 'as useful as a chocolate teapot'. There's also 'a waste of space'; I even heard, the other day, 'a waste of DNA'.
Sorry dear teachers for the meddling. But how about: They put me in my place?
But you can be put in your place without being useless, and sometimes a person put in their place is not useless at all - it just means that someone thinks their intervention is inappropriate or that they have insufficient experience: 'I suggested that they mail back straightaway, but they soon put me in my place. If I'd been working there as long as they have I'd have known that sending mail was a really bad idea.'
And yours (your intervention, that is) wasn't.
He's as much use as a chocolate teapot, or chocolate fireguard.
I also heard, as a term of address "You abbreviated piece of nothing!". Sounds pretty useless to me :)
I've heard "small fry".