From The Trade of the Century: When George Soros Broke the British Pound at Priceonomics:
In 1992, George Soros brought the Bank of England to its knees. In the process, he pocketed over a billion dollars. Making a billion dollars is by all accounts pretty cool. But demolishing the monetary system of Great Britain in a single day with an elegantly constructed bet against its currency? That’s the stuff of legends.
A small tax on currency exchange would be a practical measure.
It's a very good article, by the way.