If it's in the chat: Click their name, then 'Report' in the popup.
If it's via the mail system, take a screenshot of the mail as evidence, then right-click anywhere in-game, click 'Contact Support'.
Once there, fill out the ticket details and click send. You should be taken to a screen showing all tickets sent, click the one you just made, and when it opens, you should see a green button to the right 'Upload Attachment'.
Click that, browse for your file, upload and send.
The moderation staff will then investigate and take any necessary actions should they find rule breaking.