A little AppleScript to remove notes from any of your contacts that have them. You could also use contains "string" instead of is not qual to "" to remove specific notes.

gistfile1.txt

tell application "Address Book"
	set num to 0
	repeat with i from 1 to (count every person)
		if (note of person i) is not equal to "" then
			set note of person i to ""
			set num to num + 1
		end if
	end repeat
	display dialog "Removed " & num & " notes." buttons {"OK"}
	save
end tell