You guys should let your voices be heard and leave not only negative trust on his profile, but also PM Theymos requesting he receive a scammer tag. Theymos is incredibly busy, but he will respond and do the proper thing.
As I understand it, scammer tags are a thing of the past -- they were replaced by the trust system.
The trust system, and the default trust, have been discussed to death already -- pretty exploitable, considering TF was on the default trust.
Finally, giving Ken a scammer tag will accomplish nothing -- he already has the coins, and is well into the endgame.
Theymos doesn't like getting involved in this shit, which is understandable -- if I was in his shoes, I wouldn't touch stuff like this.