All I ever heard was speculation, never saw the cheats for myself. I seriously doubt they were real though. NOLing was bad and all but it's only data damage. VK and DK were property damaging and I doubt they existed
Saying that, my Dreamcast did sporadically reset on any game after playing this but that's probably due to spending an ungodly amount of time on PSO :] --- "We play the game without having to worry about viruses, system instability, OS deterioration, Compatibility issues, or crappy controls." - 95_Eclipse
DC Kill for certain did exist in at least one form. That one form being the kill code that would wipe the DC's internal memory. That stripped out your saved ISP data and the stored PSO SN/AK rendering any PSO characters you had unusable again.
As for the actual "DC Kill" that would brick the Dreamcast, I met a hacker who was in the group that claimed to have developed said code. And if all rumors are true, that code was one of the factors which lead Sonic Team and Sega of America to close down the US PSO servers back in 2003.
The other factor being the "Server Reset" code which was developed by yet another hacker group and was being used quite liberally before the US servers terminated. And yes, that code did exactly as its name implies, it would quite literally reset an entire ship automatically disconnecting all players who were presently on that ship/server. And according to rumors of the day which time seems to have proven were true it was the Server Reset code usage that prompted Sega Corporate to trace IP's of the malicious code users and lead to the discovery that the very elitist group of hackers using said code all had US and Canadian IP addresses. Now in regards to the claim that all of those persons using that code were actually entirely North America based or not I'm not 100% positive considering Sega also terminated support for PC PSO around the same time as shutting down the US servers. Regardless of what the case was, the combination of terminating US PSO and PC PSO's servers did put an end to anyone using the server reset code and also brought an end to people claiming that they had fallen victim to "DC Kill".
VMU Kill was also very much real, but very rarely did you hear of it happening. Most of the time hackers would use the easy codes to get a hold of such as "NOL" and the save file corrupt codes, both of which could ruin your PSO character save if you didn't know what and how to perform the standard stop, drop, and roll procedure immediately. --- Welcome To The Next Level---SEGA http://www.segaforums.com/hosted/psoarchive/