I don't think the message was game breaking in its content. A warning should be sufficient for now, especially for players who have not participated in no diplomacy games before.
I thought that all the participants should know that some game administrators can be very strict about such things. It's a good idea to pay close attention to the game rules before you join.
If I recall correctly, the player in question has tried to use diplomacy in a no diplomacy game at least once before and should know better. Let's all please try to keep to the "gentleman's spirit" (as sexist as it is
) of the game.