I used exiftool for batch removal of keywords for a long time, but recently it stopped working. I could still overwrite keywords, but as soon as I deleted the keyword, the old ones were back. After examining the binary files directly, I found out that at least Aperture writes the keywords as XMP Subject, and that Preview falls back to XMP Subject if there is no IPTC Tags. That's why when I overwrote the keywords, it seemed to work: XMP Subject tags were still there, but were not displayed by Preview.
You'll need to install exiftool for this to work.
The command I now use is:
exiftool -IPTC:Keywords= -XMP:Subject= [a-zA-Z0-9]*.jpg ; rm -f *_original
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