If you are the editor of a conversation, you can delete any message block. If you are a guest in the conversation, you can only delete the blocks you have created yourself.
To delete a block, press the small cog icon in the bottom left corner of the message block to delete.
Press 'Delete this message', followed by 'OK' on the confirmation pop-up, to delete the message.
Deleting a block permanently deletes all its content, including its audio. Note: this action cannot be undone.