I know of many words to insult people and call them stupid, but a majority of them are incredibly offensive. There are more polite phrases, such as 真笨, which I've seen used within families (as a plain 真笨啊！）, but nothing which seems the right level of formality for the context in question.
In English, the nearest equivalent of what I'm looking for would be along the lines of "fool", but not quite as strong as "imbecile". Something you can use in a formal situation, but something that won't incredibly offend the listener; more a "silly you" with a higher register of formality.
I'm not sure which words match this closely, if any exist. What words are closest to this meaning?