The most famous example of a boycott effecting social change is South Africa. It took several decades, but more and more, the world turned its back on South Africa over its racist apartheid policies, increasingly refusing to buy its consumer goods, getting it ousted from the Olympics, among other things, until the country finally abandoned apartheid.
So, boycotts can work, and some think that might be the best way to make some changes in the U.S. as well.