First of all, is this really a problem? The most severe rule-breakers are perfectly aware of what they're doing. Someone who accidentally broke rules rarely get more than a warning.
I suppose it would help with the texture thefts.
If this were to be implemented, which rules would you pick? Keep in mind nobody wants to read through a huge list when they're about to play a game, so it's good to keep it short and understandable.
I personally like the popup idea, specific to when you first enter multiplayer. The absolute majority probably go single player for a good while before even clicking mp, so forcing rules on players the first time they launch the game is a terrible idea.