Simply click on the opponent's username, look for the button on the user's profile that, when you mouseover it, says "Report [username] to Moderators".
Click the report button, and it'll bring you to a page where you can submit a report. Select a reason from the dropdown box, copy and paste the conversation in question, and click the send button.
Please help the moderation team by providing a translation of the conversation in question. It'll speed the process up. The moderation team will look over the report and take appropriate action.