I experimented a bit with this when "make an offer" was introduced. I mock-listed something, getting as far as preparing it all, and then didn't hit the LIST button because it was only a dry-run.
You can specify a minimum below which you will not even entertain an offer, and that means completely ridiculous offers are automatically not even passed on to you; they're automatically rejected.
As far as preparing a succinct reply is concerned for such offers... what about this?
Thank you for your interest in this high-quality item.
Unfortunately your offer does not reach the appropriate minimum for this item.
I will be happy to consider any reasonable offer.
Best regards,
BNWT