You can do this by going to Home, then Account, then Privacy. It should take you to this page: http://www.chess.com/home/privacy and you can add the username(s) of the people you'd like to block near the bottom of the page.
Blocking certain users
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How do ~I block someone?
I don't ever want to play Joseph Hubler again, so how can I fix it so that he can not accept any of my open challenges?
Most sites have this so that lower forms of life get gradually weeded out.