Every now and then, I get spam in Messages on OS X. I assume that the spammers just try addresses at random; or they may have harvested a bunch of mac.com, me.com, and icloud.com addresses and target them. Lately, I’ve been getting one or more a day.
As you can see above, the spammers send links, hoping you’ll click them. You’ll either end up on pages asking you to log into something, or the web pages could serve malware directly to your Mac.
These are annoying, but it’s easy to block these people to ensure that you don’t get any more messages from them. Right-click on an avatar in the sidebar, and choose Block [username]. This tells Messages to no longer accept messages from that user. You can block users who contact you by sending iMessages, or who send you messages over AIM.