You should troll them back while keeping yourself within the rules. I love getting that sort of message and if you can get them angrier then it just gets funnier.

As for what WG will do. Not much. Since all reports are cumulative then unless he has history then he is unlikely to be sanctioned. And if he were then you wouldn't know. Otherwise he may get a chat ban. I know of one occasion when someone go a game ban, just the one.