Student or Learner
1- You are nobody but an idiot.
2- You are nothing but an idiot.
I have problem which one is OK - nobody or nothing. It seems to me that both could be OK but cannot make sure of it.
Only "nothing but" works here. It's a set phrase, meaning "only".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.