This guide will show you how to add Blocked Senders:
In your mail section, you can block users very easily by selecting a message, clicking the extra options drop down, and selecting the 'Block Sender' option:
A popup will appear that will ask you to confirm your choice. Select Block:
Now, you will be able to see this blocked user, and add to your blocked senders list in a much easier way.
You can go to 'Settings' and then 'Content Filtering'. From there you will click the Internal Blocked Senders entry:
After opening this you can make changes to the conditions and add email addresses to the 'From Specific Addresses' list and domains to the 'From Addresses' List: