Similar experience with Cancellation of return after outbound no show,except I did notify before I even booked. The rep didn't explain the consequence just said it was my choice. What a joke.
It cost me 2000 Eur. Under the TOC they have to at least refund my canceled leg. But you can't do this through telephone or online. You have to write a letter or send a fax (haha) to
Lufthansa Kundendialog in: Deutschland
Name
Deutsche Lufthansa AG
Customer Feedback
Stadt
Frankfurt
PLZ
60492
Adresse/Postfach
710234
Adresse
Germany
Telefax
What a joke. The truth is they misrepresented the contract and that's a legal breech. So I wrote to their ombundsman.
You guessed it he referred me to the address above. I know where not to book next time.