EFNet just added this exact feature, but it's usermode +g
//mode $me +g
Blocks all private messages, notices, etc.. and warns you via /notice if someone is trying to contact you... unless you're being flooded. I'm guessing if you're being flooded while you're +g, it alerts opers present and aids them in k-lining them, since it would be obvious they're blindly messaging someone who doesn't want to be messaged.
