How can I delete Playstation messages?

If I leave a chat with another player on Ps4, can the other player still see the messages? Or is it somehow possible that the interlocutor can no longer see the messages?

I think he'll still see it then

What if I block it?

Then the chat will be deleted I think, so the best thing you can do is to write to Sony personally via customer support and ask to delete the messages (if they persist)