I was thinking of a very specific scenario: Suppose we have a new organization, let's call it "WikiLeaks2", which receives donnations mostly in the form of Bitcoins. Suppose a national government tries to shut it down by disrupting its reception of funds, so they set up a number of miners with the code modified so they check each transaction against a list of black-listed Bitcoin addresses (among them, that of WikiLeaks2) and automatically reject a transaction coming to/from those addresses.
Let's assume this government has successfully set up moles deep within the organization, so any new reception address is known by its agents after a few minutes. Would they need to control 51% of mining power to make it impossible to send funds to such an organization, or would disruption be possible with a fewer percentage?
Which begs the question: who validates the acceptance/rejection of a transaction, done by the miners, is correct? I.e. who validates the validators?