I've never made a melodramatic song and dance about initiating a boycott, but there have been some games or companies I've stopped patronising after unsatisfactory experiences.
For instance, after buying and being thorughy unimpressed with Dead Rising and Call of Duty 4: Modern Warfare, I haven't bought another game in those series, also after a sting of shoddy products and poor customer service from Sony, I haven't bought another Sony product since my 6th and final PS2 died.
However, does it really count as a boycott if you just don't want it in the first place?
Although my experience with Sony has made me not want to buy from them again, the main principle behind my avoidance is self-serving, i.e. I don't want to spend my money on products that I think are inferior, prone to failing and come with poor, accusatory customer service.
Sony have yet to release a product that I want, but hypothetically, if they did release a new console, TV, camera or phone that was unequivocally the best one of those products that I could buy (and I needed), then I may upgrade my stance of not buying their products from self-serving disinterest to principled sacrifice... although my resentment of the "Sony Guarantee" would still mean it was self-serving.
I suppose buying Fair Trade products is almost like a boycott of all non-Fair Trade products, but until you make a big song and dance about it, it's not really an official boycott.