I too have accidentally added keywords to several of my images, but when I've tried to remove them using the Find & Replace, I've encountered a few things.
1) Often times the Replace button is dimmed, and the only thing that I can do to make it work is to Quit PM. I've tried checking and unchecking all of the options, and it still doesn't activate.
2) When I run the Find & Replace, for instance, looking for "xyzzy, " I receive an error of "The search term(s) were not found or an unrecoverable error occurred. Nothing was replaced." Yet, I KNOW that the term exist, as I can see it in the image's keywords. Even if I Copy & Paste the term from Keywords, into the Find & Replace field, I still get the error.