I haven't had this happen, so I can't tell for sure, but can you go into the memory section on the system tab and delete the save for the King Kong Demo? If not, I bet you could somehow remove any trace of it from your hard drive using the same method that the people are using to get XBOX 1 saves on their drives. I'm not sure that this would remove it from your tag or not, as the live servers may keep a record of what you've ever played, but you may be able to, if they are refreshing your achievements from your hard drive every time you connect.
As I typed this, I realized that in order to track what you've done wherever you go, or if you got a new hard drive at some point, or whatever, they would have to be storing this on the live servers. So I would say, I'm pretty sure that there is nothing you can do to make live lose its record of you playing the demo. Maybe the live admins will clear them out at some point.