There are a couple of ways, you can either flag the rude comments so they go directly and immediately over to a moderator, or you can email the folks over at customer service and give more detailed info, such as the member's name and links to the rude comments. You can reach customer service here: http://asksupport.custhelp.com/app/ask
If you're seeing rude things, you can report the specific answers/comments that are being rude and we can deal with them as necessary. If you want to report the user themselves, you need to do so from their profile page. If you're in the browser, there should be a 'Report as' link located near their profile picture.