For starters.....how about a "rip-off" page on their website. This could show examples of the many Zalman copies.....complete with names. They could further explain why the Zalman original was superior to the fake.
But for every person that reads that page and says, "Ok, now I know that those are knockoffs and I shouldn't buy them" there will be at least one person, and probably more that says, "Hey, that knockoff performs 90% as well as the original, but costs 1/3rd as much. I don't give a rat's ass about Zalman's copyrights, I just want a heatsink that works for as little money as possible. Thanks Zalman for telling me where to buy them at". That's why you almost never see any company make mention of knockoffs of their products...it's like free advertising for the scammers.
What they probably should do is start threatening the US-based retailers. There's not much they can legally do to stop them, but an offer of "Stop selling the knockoffs and we'll let you sell Zalman" might be a start.
For many companies the way to deal with knockoffs is to funnel money in to development. Suing them is like playing whack-a-mole, but if you continuously upgrade your products the knockoff companies can't compete.