Hacked DVRs again?! (Probably the wrong forum, but probably of interest nonetheless)

So, I run a small chat service and it has attracted abuse from multiple kinds of open device. Most recently, I've found DVRs being spammed through. This is the kind of "default password"/"open Cisco" abuse that is very hard to detect with an open proxy scanner without, well, logging in and seeing if you get a shell. Has anyone ever seen this? What can I do to prevent it?