After testing, it seems that the report and ignore feature works as expected:
- After having been reported, you cannot send a new message to the player. You will get a pop-up saying that this is not possible.
- The ignored player can still send messages in the existing conversation (this isn't blocked).
- If you delete the conversation, you will not receive messages from the other player anymore (even if they keep answering to them).
So, if you don't wish to receive anything more from the player after ignoring them, you should delete the conversation that you have reported.