Perhaps the first question to ask is whether his "PR war" is justified.
Actually, you're confusing the issue. It's fine to debate the environmental impact of atrazine, but that's not the question at hand. The issue is *the ground rules of a fair and civilized debate*.
I can't punch you in the mouth to shut you up, just because you're wrong. A civilized debate allows the wrong side to make its case without harassment, because freedom to have only "correct" opinions is no freedom at all.
So, the flipside of that question is: what should companies do against persistent but scientifically baseless attacks? Almost anything they can do can be twisted around to make them look even more manipulative and guilty.
Seriously, you don't know the answer to this question? You can't see a more appropriate response than a campaign of dirty tricks and character assassination? Well, lets start with what they shouldn't do if they want to avoid looking manipulative and guilty: they should't harass people and invade their privacy.