With the release of OS X 10.9.2 Apple added the ability to block message senders in the Messages app just like you could already do on the iPhone and iPad. The feature is tucked away in the Messages preferences, and easy to use once you know where to look.
To block a sender on your Mac, do this:
- Launch Messages and go to Messages > Preferences > Accounts.
- Select your iMessage account from the list on the left.
- Click the Blocked tab.
- Click the plus button at the bottom left of the blocked list, and then choose the names you want to block. The list comes from your Contacts, so if they aren't there you'll have to add them before they can be blocked.
Block and unblock senders in Messages' preferences
You can remove people from the list, too, any time you want. Just select the entry you want to unblock and then click the minus button at the bottom left of the blocked list field. The downside is that if you have multiple contact points for someone, like an email address and phone number, you'll have to unblock each individually. The upside is that you can selectively unblock contact points for individuals.
This is a handy tool for stopping messages from specific people, but it isn't a great way to temporarily silence incoming messages. If that's your goal, check out Melissa Holt's awesome tip on using the Do Not Disturb feature in OS X Mavericks.