The effects of other people can really linger in our minds. If you no longer want contact, a simple reply of something like ‘I don’t want to revisit our past, or have any further contact. Please don’t contact me again’ and then block number. Message is useful because it’s closure for you and sets a clear boundary, but if you think it’s better for your mental health to not reply at all, just block number and say good bye to that part of your life.