I still do not see a button for this. If its there its well hidden. I am not trying to delete my own messages, I am trying to delete unsolicited conversations to declutter the DM's.
Discussion
On the client, you can only delete your own things for now. But on your inbox relay, you can do whatever you want. That's why it is so important to talk to them.
We will develop features to hide events from others but if they are not erased from the relay, over time they will just consume more and more data.