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:
Now, you will be able to see this blocked user in your blocked senders list.
You can go to 'Settings' and then 'Spam Filtering'. From there you will click the Blocked Senders List:
After opening this you can add senders, one per line, and hit okay to save. Then hit 'Save' at the top left corner of the settings screen.