In order to block someone on your Mac, open the Messages app -> click on Messages in the menu bar -> Settings -> click on the Accounts tab -> find the “blocked” tab and click on the small ‘+‘ icon to select the contact you want to block.

Note that both, their phone number and email address, will be added to that blocked list if you have both kinds of information stored in you contacts. In order to delete either or both, simply select it and click on the ‘–‘ button.

No receipt will be sent. A blocked user won’t actually know that you’ve blocked or unblocked them.

If someone keeps bugging you, but you don’t actually want to block them; you can also turn off notifications or schedule a specific time for turning off notifications with the built-in Do Not Disturb feature of OS X Mavericks.