The boycott is related to something trendy and in the news. For example, SOPA and PIPA. There are tons of vulnerable companies in that support SOPA and PIPA
The company is a consumer goods and services company. Mining companies are immune from consumer boycott pressure, unless it is a highly visible item like diamonds.
You are a current customer or buy at least occasionally.
The industry has a variety of at least similar competitor. Presumably you’ll still need to get your stuff form somewhere.
Bad behavior isn’t widespread or fundamental to the business. Oil companies come to mind– they all pump huge amounts of carbon into the air and they all have spotty records when it comes to pollution from extraction and refinement. If the bad behavior is fundamental to the business, then its the consumer that needs to change. Again until people stop driving everywhere, there isn’t much point in boycotting an oil company on account of it’s carbon pollution.
There is a way to let the company know they are being boycotted.
The company has an effective way to communicate that they really have stopped their bad behavior so that consumers can resume buying. The Nesles Kills Babies campaign had effects long after the company said “uncle” and tried to respond to the campaigns demands. If the company doesn’t notice and think that responding to the demands will lead to normalization of sales, they might just treat the boycott the same as bad luck and bad weather–bad, but nothing they can do anything about.