Why would you want to ban sledging? Not appropriate at a lower level or with youngsters but we play against certain players who we know we can get them out just be giving them a few words early in their innings. Just as an example, bloke batting for the draw this weekend 15 off about 100-110 balls, i said id give him a fiver if he can hit one off the square. The following ball he runs down the wicket thick outside edge for a caught and bowled. He did complain to both umpires upon being caught about the comment but they told him to 'find some mental strength'. He wouldn't have moaned if he had hit it for a boundary....
Its hard to know where the line is, exactly, obviously, but as a rule of thumb I'd say that if its a joke that is shared between batsmen and fielders, then its banter, if its one-way traffic designed to annoy/upset/antagonise the batsman, personal insults that are thinly disguised as "humour", then its sledging and it really isn't welcome at any level.
As for the example you give, then it depends how it was said and in what context. If it was said once, with a smile, then that's fine and the bloke is clearly a bit of a crackpot for allowing that to put him off. If a similar comment was made EVERY BALL, in an antagonistic manner, then that's probably crossing the line and ruining the bloke's Saturday, and hence sledging.