Naturally, the obvious answer is to refuse to buy anything for people you dislike, but if you could just opt out without causing some sort of catastrophic melt-down you would have done so years ago. And the other obvious answer is the most generic gift-card-ish thing possible, but think how much more satisfying an option passive-aggression in a gift bag is.
The key, with passive-aggressive giving, is subtlety. You want something that makes them uncomfortable, but which they can't admit makes them uncomfortable. Or you want something that insults them, but so subtly they don't realize they've been insulted. Or you want something that makes their life more difficult even while it suggests that you've been incredibly thoughtful. (Which you totally have! You thought long and hard about the worst possible item to inflict upon them, didn't you?)
The first year I wrote this, I noted that OxFam Unwrapped let you give people crabs, worms, and shit (as well as cuter things like sheep, pigs, and children's school supplies). Sadly, they have gotten rid of the crabs and worms, though they still have shit. (That first year, the question was raised about whether they had something that would, in a sense, constitute a gift of coal? We settled on a stove. Last year, you could actually search for "coal" on the OxFam site and they would link you directly to this very nice energy-efficient stove. This year, "coal" no longer turns anything up. ALAS.)
Anyway. Onto the new suggestions.
1. Something that they know they WOULD use, if only they were a better person. Like exercise equipment. It doesn't have to be something huge like a stair climber (although if you are passive-aggressive and also really wealthy, a stair climber is a fantastically bad gift) -- a yoga mat in a nice carrying case will work just as well, and can be had for $20 or so at Target (you'll want to lose the gift receipt, of course). Every time they see it, they'll feel guilty. Getting rid of it involves admitting that they're never going to use it, so it'll keep sitting there, in the corner, working its magic (which is to say, taking up space and gathering dust). If they actually do yoga, this gift still works, because in that case, they undoubtedly already have a yoga mat they like just fine, and it's probably a better quality one than the $10 model from Target (I have a $10 Target yoga mat; it's usable, but it's not what you'd call a GOOD yoga mat) so this is completely useless to them (unless they're bringing a friend to yoga) and yet you look like you're really thinking about their interests.
2. A really, really thoughtful book. There are several approaches to take here. You could, for instance, give them a book that relates to their interests so directly that there is no way on God's green earth that they don't already own a copy. (Like, if she's really into Irishness, you could give her "How the Irish Saved Civilization" or maybe "Angela's Ashes." If he's a dog person, get him James Herriot's "Dog Stories." Bonus points if you know this person already owns "All Creatures Great and Small" and its various sequels, since I'm 99% sure the Dog Stories is just a collection of dog stories from those books, compiled into a new book.) Alternately, you could give them something that's kind of nauseatingly mundane and sentimental ("Chicken Soup for the XXXXXXXX Lover's Soul"), or something that relates to their passion but gets it ENTIRELY WRONG. To hit the target perfectly, you might have to find someone else who's into this topic and find out what books you would definitely wish to avoid to learn more about it.
3. A cutesy, overly specialized kitchen gadget. I just discovered that Amazon.com has an entire section devoted to this sort of thing. What you want is something that they will forever feel like maybe they'll use next week, so that it will sit endlessly on their counter, taking up space, gathering dust, and making them feel guilty like an unused yoga mat. Look for something bulky. Although in two minutes of browsing, I found an electric fortune cookie maker which has got to win the prize for the silliest gadget EVER. It might actually cross the line from "passive-aggressively evil" to "so pointless it somehow becomes AWESOME," though.
I'll note that if these people have a small child, and you get them a kitchen gadget that makes something that the kid likes, the kid will never, ever let them discard it. And will nag them endlessly to use it. So definitely in that case go for something the kid will adore, because you like the kid, right? even if you hate the kid's parents.
4. An awful keepsake ornament. Speaking of things that kids will love and their parents will hate, Hallmark makes a "big mouth billy bass" keepsake ornament.
5. A gift card to a restaurant they'd never want to eat at. There are all sorts of reasons that someone might not want to eat at a particular restaurant -- you don't want to give and Outback Steak House gift card to a vegetarian, as that's not at all subtle. But if they live in a city and greatly enjoy all the unique and interesting offerings of the creative local chefs and interesting ethnic places, you can surely think of a restaurant chain that would horrify them. If they live in the suburbs and would LOVE a gift card to Outback or Red Lobster, give them a gift card to a great place in the city that has NO PARKING LOT. If you check the restaurant's website, parking info is usually under "location/directions" and if it says just "metered on-street parking nearby" that's perfect. (It's even more perfect if the restaurant's in Minneapolis, because Minneapolis just replaced all its old coin-op meters with fancy new ones that take credit cards, and what this means it that you pull into a numbered space and then have to enter the space at a kiosk.)
As a bonus, some restaurants have deals where you can buy a certain amount of gift card and get an extra gift card as a bonus, so you could potentially choose a restaurant you WOULD like to eat at and pocket the extra gift card.
(Prior editions of this gift guide: Gifts that say, "I had to get you a gift. So look, a gift!" and Beyond Fruitcake: Gifts for People You Hate.)