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

[Large Hadron Collider <large.hadron.collider () gmx com>]

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?