A really powerful entity, let's say Facebook, or the US government would find it difficult to do any of these with Bitcoin (used as the example because it's well-known):

* Alter the monetary policy significantly

* Prevent a specific person, organization, or country from using the currency

* Prevent the use of the currency for specific goods or services

That doesn't make using Bitcoin a magic bullet to be free from the influence of the US government even if its popularity and monetary policy were appropriate for widespread use as a currency, but it raises the difficulty level versus influencing someone who's doing business in USD.