The sugar boycott around 1791 is one of the earliest examples of consumers using their purchasing power to reject the trade in goods which have not been ethically produced. If we can learn something from history (and I constantly seem to have grave doubts as to what, if anything, we do learn from history) then the idea that the mobilisation of the public remains an essential tool in achieving political change is still relevant today.
I'd go further and suggest public mobilisation is a very meaningfull tool in creating change at the company level and that companyu change can be quicker and as impactful as goverment/political change.
The story of the sugar boycott (h/t Mike Kaye from the BBC history website, now archived)
In 1791, thousands of pamphlets were printed which encouraged people to boycott sugar produced by slaves. Estimates suggest some 300,000 people abandoned sugar, with sales dropping by a third to a half. Some shops advertised goods which had been produced by 'freemen' and sales of sugar from India, where slavery was not used, increased tenfold over two years.
Hundreds of thousands of people also signed petitions calling for the abolition of the slave trade. Many supported the campaign against their own interests. For example, in Manchester (which sold some £200,000 worth of goods each year to slave ships) roughly 20% of the city's population signed petitions in support of abolition. The size and strength of feeling demonstrated by these popular protests made even pro-slavery politicians consider the consequences of ignoring public opinion. One pro-slavery lobbyist of the time noted that the 'Press teems with pamphlets upon the subject ... The stream of popularity runs against us.'