I've seen Devil, or whatever he decides to change his name to, in the PT server ONLY. I feel bad for you guys that run your own servers and have to deal with him...
Never seen him in my server though. Scripts are easy enough to write for HL, I've written and used them in Counter-Strike to buy weapons with the press of a button, and in DMZ to do the special attacks (crappy mod =P).
My guess is that he's changing his name to a single character that HL does support (it happened in counter-strike until 1.1, I think) and it crashes the server. If you run your own server, just having LOG on, and then get his wonid after he crashes your server, and post it here so everyone else can ban him too. Thats about all you CAN do, until the ESF team fixes it.
On a side-note, about the power of admin-mod: YOU CAN'T DAMAGE SOMEONE'S HARDWARE WITH IT. You're not able to make electrons go where they aren't supposed to. Yeah, you can unbind all their keys, or make their framerate **** up, or make them quit HL, but you've gotta remember: Admin-mod only has as much sway on your computer as HL does. It can't shut your computer down, or turn your furnace on, or open your front door.