Yes, actually I have.
I boycotted KMart for a season while they were dealing with pornography issues in the 90's. I had to shop at another store that was way out of my way...but that didn't matter.
I won't tolerate pornography.
For the most part, I do not participate in boycotts, at least purposely. There are many corporations, companies, and organizations who I find fault with, but for the most part, I do not use their products to begin with, which would be better labeled ethical consumerism rather than a perpetual boycott, or there is no superior alternative, assuming that there is an alternative.
If, for instance, I find some of the practices of my specific Internet service provider questionable my options would be to look into another Internet service provider. There are two other major Internet service providers in the area. Both do not offer the same service as my original, and while their specific objectionable actions, at least those which are known to me, are not the same which I objected to under my old Internet service provider, they are no better. Also, at the current time, I do not have an Internet service provider, and I am currently connecting through my college's network, which uses the same Internet service provider that I was previously using, and I intend to resume using.
There have however been occasions where I have participate in a boycott. When TV land took Get Smart off the air, I did not watch the channel for over a year. The other notable incident had a far greater effect, but I will not right of it now.