It would be pointless you taking legal action - you don't have a "cause of action". You have 2 alternatives:-
1. (Which I strongly recommend) send them a recorded delivery letter (keep a copy) enclosing a copy of their letter confirming your withdrawal from the University and pointing out that you therefore do not owe them fees; or
2. Do nothing. They will then continue to harass you and will either give the alleged debt to a debt collector (who will also harass you) or start Court action - which you will have to defend. The Judge would probably ask you why you did not resolve the matter earlier by letting them have the copy of their letter - you would not be very popular with him/her as both parties to any dispute are supposed to take all reasonable steps to settle before going to Court.