Assuming you are still in the old email format-------------
From your Inbox page, go to Preferences>>>>>>Email>>>>>>>>Email Filters. Click on that, and then follow the prompts to set up a filter for that sender. One of the options is "Discard", which dumps any emails from that sender and you just never see them.
If you are in the new format, click on the gear icon in the upper right corner of the screen, then on Settings. There you will see Filter Rules. Click on that and follow the prompts. In the new format there are basically two ways you can block someone; first, you can just pick "Discard" as the action same as with the old format.