Creating footnotes

I found out accidentally that if I have

calibre appears to try to make zzz into a footnote, although the footnote is broken. If I create an azw3, in the book viewer I see zzzz in the footnote pane, although clicking the link does nothing. How do I

a) suppress this feature where I don't want it, and
b) create a footnote for real when I do want it?

thanks