Anyway, from this point on the following fandoms are considered out of bounds for slash:
7th Heaven, Alias, Alien Nation, American Gothic, Andromeda, Angel, Babylon 5, Backstreet Boys, Batman, Battlestar Galactica, Beauty and the Beast, Big Wolf on Campus, Birds of Prey, Blake's 7, Charmed, CSI, The Dark is Rising, Doctor Who, Due South, Earth: Final Conflict, Dune, Elfquest, Emergency, The Equalizer, Fastlane, First Wave, Forever Knight, Good V Evil, Gundam Wing, Hard Core Logo, Harry Potter, Hercules, Highlander, The Invisible Man, JAG, John Doe, Kung Fu: The Legend Continues, La Femme Nikita, Lois & Clark, Lord of the Rings et al, The Lost Boys, The Magnificent Seven, Miami Vice, The Man From U.N.C.L.E., Millenium, Mutant X, Nash Bridges, N'Sync, Once a Thief, Pirates of the Caribbean, Poltergeist: the Legacy, The Pretender, The Professionals, The Profiler, Quantum Leap, Remington Steele, Roswell, Seaquest, The Secret Adventures of Jules Verne, The Sentinel, Simon and Simon, Sliders, Smallville, Space Above and Beyond, Sports Night, Star Trek: TOS, Star Trek the Next Generation, Star Trek: Voyager, Star Trek: Deep Space 9, Star Trek: Enterprise, Stargate SG-1, all Star Wars movies including Phantom Menace, Starsky and Hutch, Swamp Thing, Superman, Third Watch, Twin Peaks, The West Wing, Wiseguy, Witchblade, Xena, The X-Files, The Young Riders.
And, of course, so many, many more--did you know, for instance, there are 2928 stories on FanFiction.net about the movie "Newsies"? Let's hope none of those is slash. You can find more off-limits material here, here, here, etc.
If I were doing this correctly, I'd make a list of all the straight characters, naming each individually, but in many cases I found it's surprisingly easy just to list the show itself--because none of the characters is gay!
Permitted to "slash" as long as you're not using other, canonically straight characters:
Jack (Dawson's Creek), Inara (Firefly), Dorian (Eroica), Chiana (Farscape), Northstar (X-Men), Bayliss (Homicide), those guys on Oz and Queer as Folk, Willow and Tara and Kennedy (BtVS), Cindy (Dark Angel), that nice couple on Northern Exposure, those Velvet Goldmine pansies, and probably that chick on Lexx.
As you can see, it's a rich area for writing, and we haven't even listed all of those obscure queer characters, who can be so much fun to flesh out. "Canon" and "gay" don't have to be mutually exclusive!
If I've got any slash in my peanut butter, or if there any characters or fandoms you'd like to see added, please tell me and I'll edit this list. Or then again, maybe not.