To boycott means to stop buying or using the goods or services of a certain company or country as a protest; the noun boycott is the protest itself.
This noun comes from the name of Charles C. Boycott, an English land agent in 19th century Ireland who refused to reduce rents for his tenant farmers. As a result, the local residents did not want to have any dealings with him. Boycotts are an effective way to use your spending dollars to affect change.