Well, I guess as a radical approach you could do something like replace every other space with an "i" or some other narrow character, but put it in a font color that's the same as the background color. (You can't do every space, obviously, because then it won't word wrap.) So it'll look normal, but when you try to copy and paste it, you'll get something like this:

Thisiis aiwonderful product.iYou shouldibuy itiimmediately.

Of course, an actual customer who tried to copy and paste the text (say, to email it to a friend who might be interested in the product) would probably get annoyed by this.