Any way to delete custom fields of cards in Contacts.app?
I was able to delete the nickname field of a contact simply by emptying it. However, here's an AppleScript that will allow you to select nicknames to remove.

tell application "Contacts" 
  -- Get a list of people who have nicknames
  set ListOfNicknames to people whose nickname is not missing value

  repeat with x in ListOfNicknames
    -- For each person, ask if their nickname should be removed
    set dialogResult to display dialog ¬
      "Remove nickname \"" &amp; x's nickname &amp; "\" from " &amp; x's name &amp; "?" ¬
      with icon caution ¬
      buttons {"No", "Yes"} ¬
      default button "No"

    if button returned of dialogResult is "Yes" then
      set nickname of x to missing value
      save
    end if    
  end repeat
end tell


You should be able to easily modify this if you want to delete all nicknames, or handle a different field like the prefix (known to AppleScript as the title).

Thanks:
Mac OS X Tips, for showing me the save command that I was missing;
The AppleScript Documentation, for existing;
And @KevinSayHi, for giving me a chance to brush up on my AppleScript.