What can you do if you know that somebody use Rybka in his games?
Report that somebody to chess.com at
---> Help (you find that in the top right corner of this page)
---> Contact us
http://support.chess.com/index.php?_m=tickets&_a=submit
---> Report suspected Cheating
Fill in that form, putting in all the useful information you can provide on this. Chess.com will look into it, check this player thoroughly and take appropriate measures if he is found to be cheating.