I consider that sort of thing to be blantently disrespectful, but others obviously were not raised with the same sort of vlaues.  I'd pull the signs and give them back.  If he asks why, I'd simply tell him the signs were inappropriate since they had his name on them when they were not his fields.  It's probably best not to be overly dramatic about it.  Just be as firm as needed and no more.  He will not likely do it again if you get your seed elsewhere.