The government cannot force anyone to accept a given price for bitcoins. That is the equivalent of the government setting the global price of gold, which is impossible, but even more difficult as bitcoins are anonymous and transactions are even harder to track. This scenario could never happen.
This is exactly what happens in China, the CNY is effectively pegged against the USD.
My point is more about the actual immunity to inflation that bitcoin is supposed to have rather than government control. Bitcoin is open source and any alt coin that gets significant traction is going to dilute the digital money pool, thus leading to inflation. The immunity to inflation assumes that there is a fixed supply of currency, but as anyone can make a new currency then surely the theoretical possibility for inflation is actually infinite.