In Notepad++ I used the Regular expression search for [^\x00-\x7F]+ and replaced with an empty string.
I don't know if it works for all extended characters but has always worked when I needed it
I should point out at this point that I know nothing about Regular Expressions and found the technique on line a year or so ago, but I don't know where
I am sure that someone here can explain the nuances of how it works