I noticed a massive incease much likes yours on my bounce rate experiment (and learned something about awestats counting this data not just making you aware). Where bots are doing 36k+ hits a month totalling over a 1.2gb+.
I put the + marker because I haven't looked in about 5 days but that was approx.
Most of mine is from Google Image search bot by the look of it, they seem to archive a thumbnail of all the graphics but not the whole thing. So they are hammering my bandwidth to get the images
Should be able to block it as Ty said with ip range blocking, you could block by identifier but the unknown one wouldn't be covered.