However, each and every country has a black market. And the black market plays a very vital role not only in terms of the country's economy but also in terms of the people's access to whatever is declared illegal by the current government. Foreign currencies are always offered in the black market. This would solve any government's ban on specific foreign currencies.
Not all countries have a black market denominated in a foreign currency. I doubt the Americans, Swiss, British or eurozone countries bother with foreign currencies at all. You would only trade in a foreign currency if your own currency was inflationary and retailers were reluctant to accept it as a result.