Banning bitcoin is free more advertising. Everybody is going to want to know what the government is afraid of. Usually the ones that ban it are in the countries that need it most.
This in fact would only contribute to spread the word about Bitcoin to the people who live in those countries, increasing the number of people interested in using the currency. Interest would be even greater when people perceive the advantages of using a decentralized currency. And in the end, Bitcoin being banned in those countries would not prevent the use of the currency, since the intervention of middleman is not necessary for it.