I had the same problem. This is how mine is set up and what fixed it for me. First, I downloaded the latest version of calibre (even though it said I was already using it, it let me download it and it made a difference)
Under mobi output my settings are:
1) I have mobi file set to both (old works as well - but don't use new)
2) Under enable sharing I have it UNchecked.
3) Personal Doc tag I have left blank
That's it. This removed the Personal banner for me. Good luck.