Aperture 3 Deleting a keyword
I've done this so many times and finally discovered the solution in Aperture's help.
Select the images that have the wrong keyword applied. Bring up the Keyword Controls (shift-D) and type the keyword you want to remove into the Add Keyword field (Aperture will probably auto-complete it for you). With the cursor still in the add keyword field, press Shift-Return to remove the keyword from all the selected images.