You are correct. When you block someone, they will no longer be able to see your messages, and any previous messages you've sent to them will still be visible to them. However, the conversation will still be available to you. If you want to delete the conversation, you can do so by going to the conversation and tapping on the three dots in the top right corner. Then, select "Delete Conversation".

If you are in a group conversation and you block someone, they will be removed from the conversation. However, the messages that they sent before you blocked them will still be visible to everyone else in the conversation.

I understand that you wanted to have a courteous exchange with the person, but they were not interested in that. Blocking them was the right thing to do in this situation. You don't need to waste your time on people who are not willing to have a respectful conversation.