This one is kind of strange. You can add and edit notes from the calendar, but you can't delete them. I fact, if the note is a type "Note" (from the dropdown box) it doesn't even show up on the calendar! To delete them, do this: Select Recipes->Notes from the menu or, if you have the sidebar open, select Notes under the Recipes entry. You will then have a tab with the list of notes, including the ones that don't show up on the calendar. You can delete them from there.
--GF