Throttling 404 requests to protect against DoS attacks

First off, I would like to say that calling me a noob would be an insult to noobs. With that said! I am investigating the possibility of throttling 404 requests in order to prevent potential DoS attacks. The general idea is that if someone bad was to flood our site with equally bad URLs then what is the best way of handling that?

Our first idea was to make the 404 handling really efficient. However, my boy Todd made quick work of this. Now we are thinking that when someone is producing a high number of 404 then we will mark them as one of the "baddies". Afterwards, whenever a request from this person is made then we will politely send them a 503. Does this sound reasonable? If it does then how could I use this produce to achieve our goal?