Based on the spam I get here, there is no big incentive to buy anything from whoever's business being advertised to begin with. To the extent there even is any kind of business there at all.
To illustrate, from looking at some recent deleted spam I make the following observations:
First of all, my penis is just the size it should be and it works the way it should, so no need for enlargments and viagra, nor, presumably, subsequently having to carry it around in a wheelbarrow.
With that taken care of, next, I don't have time to do contract negotiations with alleged attorneys claiming to represent rich deceased people whom I've never heard of, living in countries where I've never been.
Neither do I care about spending time attempting to claim a prize in lotteries where I never bought a single ticket.
I don't use Paypal in French either.
Then there are the bankers in Ghana that send me notices of their new e-mail address, with wild and wonderful and completely unrelated titles. Since all these notices are basically formatted the same way, the precipitated hypothesis is that there are a lot of bankers in Ghana, and all of them are getting yahoo.com e-mail addresses. Well, I don't need Ghanesian banking services any time soon. If I ever should, I'd deal with someone whose e-mail address had a reasonable resemblance to the name of the bank and the country the bank is operating in, not just some random attention-getter I found in the spam-box.
And that is just looking at the stuff that comes in a language I can read. Sometimes it is Chinese or Hebrew, and sometimes it is in some mysterious language that merely renders as garbage.
Point of all this, there is hardly any legitimate business or services of any kind advertising through this spam channel at all. Hence no one to boycott.
Makes me wonder why do they bother.