Outlook 2011 for Mac like the PC version allow users to block sender in Outlook. Mac users can block sender using the option available as part of Junk E-mail Protection.
Step 1: Open the message received from the contact that you want to block.
Step 2: Right click on the email address and copy the contact.
If you know the email address of the sender then you can skip the above steps.
Step 3: Click Tools menu and select Junk E-mail Protection from the menu list.
Step 4: In the Junk E-mail Protection screen, navigate to Blocked Senders tab and enter the email address to the blocked senders list.
You can also enter a domain name to block all emails from the specified domain.
Step 5: Click OK button to confirm and save the changes.
Also see: How to block sender in Outlook 2010