To remove a single keyword from a photo, or group of photos, first select the photos that have the common offending keyword(s), then type the keyword you want to remove in the “Add Keyword” box at the bottom of the interface. Then simply press SHIFT+RETURN. Aperture will then remove the keyword(s) from all the selected photos.
Additionally, if you know you don’t want a particular keyword for ANY photo, simply select all the photos in your project and remove the keyword. It does not matter that the keyword was in some and not in others.
Tip and photograph by Scott Bourne