Just so everyone's got the idea, if you're banning the name hackers choose ban by steam ID. You'll see everyone's steam id with their name next to it. The namechange hackers don't have any name, just a number. That's how you identify them. I've got a few before they could even cause any hassle this way.
It must be something caused by their name changing. I reckon the hack does this, it's done too quickly for it to be the actual hacker.