With this issue and most ethical issues I've encountered, it's important to realize that the work is still going to get done by someone. Don't fool yourself into thinking you're going prevent Pfizer from marketing their drug. But that's not why you do the right thing.

You do the right thing because you get to choose whether the blood is on your hands or not. You get go sleep and go to your grave with different amounts of guilt and peace. You get to tell your kids these stories and help them make better decisions. Through all of that, you might make the cost of deceptive websites a little bit higher and the message a little bit less effective. Or you might not, but that's not the point.

In short, don't make your ethical choices on the impact it will have on the world. Make them for the impact it will have on you. (The alternative will mean you'll struggle a lot more making the right decision, knowing that Pfizer will just get someone else to make the site and maybe you should still take the work and then donate some of the money to a good cause.)