Not trying to be obnoxious, but what does it matter if it is "low balling"? You have a notion of what you will accept, and that ain't it. At the same time, the offeror is trying to see if he can get a bargain. Nothing wrong with that, if a little annoying. Just tell him thanks for the offer, but that it's not close to what you will accept and move on. Or just ignore him completely.
Now if you are getting a lot of "low ball" offers at around the same price from different people, then maybe you need to reconsider your pricing.