This is something of an odd topic because there is a lot of variance between different play groups, and so I'd imagine that there will be a lot of different opinions on just what you should and shouldn't allow a player to be. Some GMs, especially ones like me, will even argue that there are no concepts that should just ouright blanket statement be not allowed, just ones that aren't always right for the game. That is what I want to talk about today.
Player/GM Comfort Is Key
The first thing you want to take into consideration when banning concepts is your and your players' comfort. It doesn't matter if one part of your play group is fine with anything and everything happening at the table, if one person is not ok with something than that concept shouldn't be allowed. The reason for this is simple: if a player/gm doesn't feel safe at the table, then they're not going to have fun or play well. By forcing them into that situation you are effectively ruining their fun. Worse, with some of the more outlandish/adult concepts that may be coming into play, you could be forcing someone into a very awkward emotional state. Not cool. So, above all, keep everyone at the table's comfort in mind when banning concepts.
What Fits the Campaign
The next big, and easy, one to handle is what fits the campaign. This is also one of the areas where you get into more gradations of banned. For example, you probably don't want someone playing a smarmy politician if your game is going to be a dungeon crawl or more action packed. Just like you might not want someone to be playing an Iga clan warrior if the big enemy for your plotline are the Iga clan.
Essentially, if some aspect of the game means something is strongly ill advised, it would fit into this type of a ban. Easy peesy, right?
The Overused/Over Abused
This is more a problem for LARPS and online chat games, but sometimes when you have a large venue you need to ban a concept not because you don't like it, but because it is overdone to the point that it is no longer special or otherwise done to the point where it just isn't believable to be happening again. For example, in the version of FRO that I Head GMed we had to ban the concept of "a peasant who found a dead samurai and took his swords" because in a month period we had nearly a dozen people take the concept only to then cause massive problems by blabbing about it to anyone who would listen IC. The end result was nearly a dozen executions of PCs in short order, or other disposals, and a stigma for the concept that led to it being banned. Shame too, I do like that concept.
The Completely Unbalanced
The final kind of ban I want to talk about today is something more for games like D&D/Pathfinder/Dark Heresy and other games where you choose a career path more than just build the person that you want to have. This kind of ban is more done for Quality of Life reasons for the GM, or if there is going to be some measure of PVP or PVP-potential in the game. Essentially with this, you ban any concept or kit that is too powerful or otherwise has some large advantage over other types of characters. In political L5R game terms a common ban of this sort is to block Imperial PCs. Why? Because they have a significant advantage over non-Imperial characters that can make things decidedly unfun for anyone but the Imperial character.
These are also the kind of ban that players will argue against the strongest. Sometimes they try to bargain to get their version of the concept through. Sometimes they make arguments based on faulty logic (Imperial Stats are in the books because players are supposed to be them!) Sometimes they're just rude about it. The key here is to have your reasons and to be firm about them. Making an exception for one person is going to cause problems when you don't for someone else, especially in your normal 4-6 player table top group.
What other reasons have you/do you ban concepts for? Anything particularly interesting? Any fun stories about it? Sound off in the comments below.