Delete is defined as:
- remove or make invisible; "Please delete my name from your list"
- wipe out magnetically recorded information
- cut or eliminate; "she edited the juiciest scenes"
http://www.thefreedictionary.com/delete
Closed is defined as:
- not open or affording passage or access; "the many closed streets made travel difficult"; "our neighbors peeped from behind closed curtains"
- requiring union membership; "a closed shop"
- not open to the general public; "a closed meeting"
- blocked against entry; "a closed porch"
http://www.thefreedictionary.com/closed
Adding delete to the subject line says you, the thread creator wants the thread to be removed from existence, to have it removed and not accessible again. The most accurate/precise definition(looking at the above two) is delete.
Adding closed to the subject line tells the people looking that you are no longer in the market. A good analogy is like closing a store. Just because it is closed doesn't mean the store will never open again(difference between closing and deleting).