What prevents me from leaving the country?
Well, until we are not talking about any particular case we can consider that if the government is providing very strict anti-bitcoin policy then it probably supposed to be a far authoritarian country. In such places, government would probably control the immigration so you won't be able to leave the country.
Anyway, argument about leaving the country is not relevant. Simply because it could be used almost in every case and in every country. Got no job? Leave the country! Wanna get a better salary? Relocate to wealthier country. But for some reason, many people can't follow this advice.
It is possible to censor cryptocurrencies. If you have to relocate to another country to use your money then it is not that censorship resistant.