To block someone in Gmail, first log into your Gmail account and click the 'cog' symbol at the top right of the page, in the drop down menu select 'Mail Settings'. Locate and click the 'Filters' tab at the top of the page. Next click the 'Create a new filter' link and a new window will open. Add the email address you wish to block in the 'From' field and click 'Create filter with this search' to continue. Now choose what you want to do with the messages sent from the blocked address, such as delete automatically, then click 'Create Filter'.
Creating a custom filter is the right way to block a sender.Click on the setting button top right corner of your Gmail and select the setting option.There you find a filters tab.Click there and enter the email you want to block and click on "create filter with this search".Mark delete button and click on "create filter".The process is complete.But I think you can use a simple plugin for this option called "Blocksender" which is easy to use and also save lot of your time.You can block a sender just in single click with this plugin.