One way to remove only keywords from all the files in a contact sheet in PM Version 5
1. Select all the images with command-a (OS X) or ctrl-a (Win).
2. Then command-i (ctrl-i) to open up the Stationery Pad.
3. Uncheck the checkbox to the left of every field except "Keywords" -- make sure the Keywords field is checked.
4. Make sure the Keywords field itself is empty.
5. Make sure the checkbox next to the + is NOT checked (Checked = append, unchecked = replace)
6. Then click "Apply Stationery to Selected"
This will erase only the keywords from the selected files, and as long as no other field had a checkmark, all other fields would be untouched.
-Mick