Use PlayerCapabilities::isCreativeMode to check if the player is in creative. The PlayerCapabilities of a player is stored in EntityPlayer::capabilities. You can use this on any instances of EntityPlayer (i.e. passed in to the method you are overriding) on the server.
In addition, in your case, I would recommend overriding Block#onBlockHarvested instead of Block#breakBlock, as the former passes in the player that broke the block in the method's parameter.