If you use WhatsApp, there's a big possibility that you've gotten a few messages from someone that isn't in your contacts. And they could possibly be spam. Until now, there wasn't much that could be done about it. All you could do was block the person, and not mark them as spam. The difference is, marking as spam tells WhatsApp that they better do something about this person as they are ruining the WhatsApp experience for their users.
In one of the recent updates to WhatsApp, you'll see that in messages from users that you don't have saved, there are two boxes below the message now. One that says "Report Spam and Block", and another that says "Not Spam, Add to Contacts". If you want to just block the person, you can still do that, by utilizing the overflow menu in the upper right-hand corner. It also shows up at the top of the conversation page when you've replied to the sender. You'll see Block and Add at the top.
It appears that this feature was added in either v2.12.12 or 2.12.30. Of course, the version that's on Google Play is 2.12.5, and not the most recent version. However, if you want to check out WhatsApp's website you'll find more recent versions, including the new material design version of the app, which is just beautiful.
WhatsApp is a very popular messaging service. In fact, it's one of the very few non-Google apps to hit the 1 billion download mark on Google Play. So there's plenty of people out there using the app, and messaging service. Facebook recently bought it for about $19 billion. Which was a bit strange at first seeing as Facebook already has their own messenger which is separate from the Facebook app, at least on mobile. So far, Facebook hasn't really done anything with WhatsApp, but we have noticed WhatsApp adding a ton of new features since the purchase has been approved, so perhaps Facebook is doing the right thing and making WhatsApp more competitive on the market. Either way, being able to mark someone as spam is something WhatsApp should have had long ago, and it's nice it's finally here.