Does he actually type leet? That's funny - to me, people who use leet speech all the time seem like wannabes.
I think Regexp's suggestion is the best. Just ask the guy to stop. If that doesn't work, fight fire with fire and use an auto-talker. You could P\/\/[]\[] this [\]0oB LÀ[\/]3r. Maybe he'll stop when he realises how irritating it is.
On the other hand, it could encourage him too