If you wish to completely remove structured keywords and start from scratch, you can delete the structured keywords file. 

On macOS, it is in the following location:

/Users/<your user name>/Library/Preferences/com.camerabits.PhotoMechanic/StructuredKeywords_UTF-8.txt

  1. From the Finder, choose Go>Go to Folder... 
  2. Enter: ~/Library/Preferences/com.camerabits.PhotoMechanic
  3. A window should open up at that location and you should see the StructuredKeywords_UTF-8.txt file there.  
  4. Drag it to the Trash.

On Windows it should be in this location:

C:\Users\<your user login name>\AppData\Roaming\Camera Bits, Inc\Photo Mechanic\

Depending on your settings, you may not see an AppData folder in your user directory.  It may be hidden.  You can either change your settings to show "Hidden files and folders", or just copy the above path and substitute your username in it and paste it in the navigation bar at the top of any Windows Explorer window.

Once you find the StructuredKeywords_UTF-8.txt file, delete it.