I've seen spiders user up gigs of bandwidth, one thing you can do is add iplimitconn which cuts out the amount of downloads allow per IP address. Running a video site, I found there were about 15 large video downloads at the same time from the same IP. This migitates that to your configuration ( it put 3 limit).

There's also mod_evasive which can block a repeated amount of connections from an IP that may otherwise bring down your whole server.