This is a major problem in countries, where the national currency is extremely volatile, such as Venezuela and Argentina. Normally, limits are there regarding the conversion of the national currency to other foreign currencies. But in case of Bitcoin, these limits can't be enforced in most cases, due to the transnational nature of BTC.
No matter how much a government dislike bitcoins and make it harder for people to use it or convert it into fiat, still they will not succeed in it 100%. Insipite of all the bans make by the governments, people are still able to use it online . Governments can only discourage the use of bitcoins by giving lots of incentives in local currency transactions but they cant completely ban it.