It is not very nice to call someone stupid but sometimes is necessary. I will telll you the way you can call someone stupid in Spanish:

-Eres estupido
(You are stupid)

Here I make list of some more words you might like to use:
-Eres tonto (silly)
-Eres imbecil (imbecile)
-Eres idiota (idiot)
-Eres un bobo (silly)
Perdedor! (loser!)
Pierdete! (get lost!)