I am currently preparing for a game of Vampire: The Masquerade. I've got a plot in the works that involves a Malkavian villain. Given the nature of the Malkavian Madness Network I want to exclude the Malkavians as an option as a Clan for my players. How can I do this without spoiling or even suggesting that this is because of the MMN so that the villain cannot (unintentionally) spy on them. I feel that if I allow Malks anyway I either have to handwave the issue but have the players turn on any Malks present for being a liability at best or a suspected spy at worst or have to alter how the MMN works in the game (which draws suspicion), while if I say flat out "no Malks" this might draw attention or conflict with players that want to play them anyway. So how should I handle this?
My vote is for the broad-strokes banhammer.
In short, don't make a big deal of it and people mostly won't notice.
Just because something exists in the world does not mean it should be an option for the players. Why not let one of them play as a Mage or Werewolf? Because it doesn't work in the world the GM has defined. Why not Malkavians? Because they're insane. Just because it isn't the reason doesn't mean it isn't a valid reason to ban them.
In addition to simply disallowing certain rules (like Level Adjustments in D&D), I've seen or heard of artificial GM limits such as:
- no evil
- no non-evil
- no lawful
- must have half levels in Bard
- must not play <these> classes/races
- must not take some ability or other
and they've all worked out well from what I've seen.
Particularly in a game where there are lots of possible character creation options, or where the chance of someone picking the undesired option is low.
You can tell people to bounce their character off you first, and leave it at that. Try to have this discussion before the first session with each person individually to reduce the info available to the group.
If no-one picks the Malk, then that's great news.
If someone does pick it, then tell them they can't take it and need to pick another. They know that Malks are banned, but at least they don't know that Malks are the only thing banned. For all they know, lots of things are.
Doesn't completely eliminate the problem of course, but it's better than nothing.
Instead of giving them a blacklist of character options they must not take, give them a whitelist of character options they are allowed to take. You might also consider to throw in some options from official or unofficial supplemental material to make the list longer.
That way any options which are absent from the list do not look as obvious. It also forces you to think about any other choices which might not work in our campaign for whatever reason which you haven't thought of yet.
When you have any doubts about any character choices, leave them out. When there are three or four things which are missing and you don't tell the players why, they will have a hard time to theorize if it is because of story, game balance, personal taste or just a red herring.
Be upfront about it.
In this game, PC must not come from the Malkavian clan.
This is a pre-condition of the game, just as "being a vampire",and using X system. All my games have a list of those with a few must and a few may. This sets up players to build character that fit within the main plot line I want to run as part of this game. After all, the PCs are the right people at the right time to interact with the plot. As a side note, I do ask all my players what they want out of the game, what they want to see, and what to avoid. After all, they (the players) are an integral part of the game.
This might give a clue that something Malkavian related might be happening at some point, maybe as part of the overall plot. So what? Your players should be mature enough about meta information.
Finally, did you ask the players about restricting clans? Did you ask the players if they cared about potential meta-game information spoilers? It would be nice for them to have a say in the game they are playing. Please, do not make decision for others even if it is for their own good.
I always try to keep in mind that the players have no idea what I'm planning for them as a GM. If the plan WAS to have them encounter a villain from Clan X, but one of the players unintentionally ruins this by deciding to be from Clan X? Well, I just change the story a bit: Now, the Villain is from clan Y.
I've never played the Masquerade, so perhaps the world is a bit too defined to do this kind of thing? Then again this might lead to some interesting surprises/mysteries, like: why does someone from Clan Y have Clan X powers? What unholy blood coupling could do this? Can the players do that? At what cost?
All that being said, I DO agree with the other answers about limiting the players: Indeed, getting to the end of a maze is boring, if you can fly.
If you're truly concerned on letting the cat out of the bag, there's a simple tactic that you can use to avoid letting your players catch on: throw in a red herring or two. If instead of just saying "No X clan" you say "No X, Y, or Z clan", or perhaps "No X or Y clan and no I or J class" to mix things up a bit. This might still set off people's tin foil hat detectors, but they'll be quickly barking up the wrong tree trying to identify what connection I and Y have, or X and J, or the significance of I and J and not I and H.
Another possibility would be to introduce a house rule that subtly but irrevocably nerfs Malks but not other clans. I don't know enough about the specifics of V:tM to suggest anything specific, but if you can orchestrate a situation where your players read the new rule, brow furrowed, before going "Hey, this makes Malks all but worthless" then you've won. Perhaps they have a signature weapon that is banned, or a signature power (besides what you've mentioned) that gets all but dumpstered? People might consider there are nefarious schemes behind the banhammer, but they'd never suspect the nerf bat.