It's satisfying to see that this function has finally been implemented! Bravo TeamLava! :-)
edit(KP) extra information:
Announcement thread
Looks like this is only offered for IOS bakery and restaurants games right now.
You will need to update version 1.7.1. for both games.
To block, click the x on their wall message and you will have an option to delete and the option to block.
You can unblock anyone that you did block by going under the settings in your game.
If you block a neighbor from you wall, and will still be in your neighbors list, so you may want to also delete them as a neighbor.