According to this site
HOW TO HIDE A SOFTWARE UPDATE IN THE MAC APP STORE
if you enable the debug menu in the App Store you then have the ability to right click and hide an update as kappy wrote.
This is the command to run in the Terminal to turn on the debug menu
defaults write com.apple.appstore ShowDebugMenu -bool TRUE;
once you run the command restart the App Store and you should see the additon of Debug in the menu bar right after help.
I've turned it on here but I have no updates so I can't test the right click to see if Hide does show up.
you reverse the above command (remove the debug menu) do
defaults write com.apple.appstore ShowDebugMenu -bool FALSE;
But leaving the debug menu on will not cause any problems.