The cry of "No True Scotsman" comes up so frequently in internet debates - especially religious ones - that a few heavyweights have tried to sort out when it's a fallacy and when it isn't. For anyone who doesn't know what that means, it usually goes like this:
A maniac burns down an airport, and in the investigation it's revealed that he considered himself a fan of obscure Victorian poets - Christina Rossetti in particular. People who have always disliked Christina Rossetti jump at the opportunity and claim that her poetry is degenerate and of course it leads to violent behaviour, and frankly her brother Dante Gabriel's work is far superior in style and content. Part of the community of Christina Rossetti fans respond that nobody who really understood her poems could show such a lack of concern for airport architecture. At this point, Christina Rossetti's detractors accuse the fans of twisting language so that any misbehavior would automatically disqualify one from counting as a Christina Rosetti fan.
As the most influential religious and philosophical blogger on the internet, I feel I must give my perspective. On the one hand, beliefs do usually have actual content - for instance, I think that a non-privilege-aware anarchist is either inconsistent in his belief or not a real anarchist - and it's reasonable to claim that somebody who lacks that content lacks that belief, whatever they identify as. On the other hand, saying but so-and-so doesn't really like Christina Rossetti is used to hide and minimize negative effects of a belief. Look at how often we hear that the thug-of-the-day isn't a real policeman, because real policemen protect and serve; or that they're not a real patriot. If you class every bad person as not part of your group, you've made it literally impossible to even ask whether your group supports bad behaviour.
Here's my solution: it's okay to say that somebody doesn't really hold a particular label if there's a clear dogma associated with the label and they're in conflict with it. It's still okay - albeit dicey - to say the same thing when there's no clear dogma, so long as you're in the majority in your group. When the vast majority of Christina Rossetti fans denounce the burning of airports and all violence, and the attitudes that lead to violence, then what they're doing is ejecting airport-burners from their community - and groups do have the right to police themselves. However, if you're doing this often enough, you might want to ask yourself why your community seems to require such frequent purges. You might want to ask yourself why your community seemed attractive to somebody with airport-burning tendencies. And if the majority of your community doesn't speak up with you, you might want to consider getting a new community.