空け者 (utsukemono: fool, idiot, dunce) empty + person
The primary way of writing this old-fashioned insult is in hiragana. After that, your next choice would be 呆気者 (to be amazed + spirit + person). And your third choice would be 空け者. Apparently, there are many ways of slinging this insult at someone. (And that person probably won’t pay too much attention to your choice of kana versus kanji.)