If iTunes can't help you, then we might solved it manually.
1. Go to the file itself on your Operating System.
2. Open the .mp3 with Wordpad
3. Search your lyrics, it should be somewhere in the beginning of the file. Don't mind all the other strange characters, for this we don't need them, so we stay of those strange characters.
4. Delete the lyrics. Mind to only delete the characters of the lyric itself, let other characters untouched.
5. Save the file.
6. Go back to iTunes and see that the lyrics are gone.
It is wise to make a copy of the .mp3 file before you start editing, in case you delete to much 😉
Let me know if it worked 🙂