In my experience with exactly what parent comment discussed... it's not effective. In fact, the company may even (which I have witnessed personally multiple times) blatantly violate your OSS license to incorporate it into their proprietary money-making product, because they know they can get away with it... most lone devs do not have the money or willpower to attack a corporation, even if they could win.

Usually I see those lone devs either ignoring them entirely, or ragequitting open-source altogether.