The previous post was quite accurate. Clarisse's value system is what defines her as "anti- social." It's a great move by Bradbury to depict how a social order perceives "the other" as different or threatening and in order to remove this threat, the label of "anti- social" is applied. In reflecting on Clarisse, it is impossible to really see her as anti- social. She tells Guy that she and her and family like to sit around and talk. This is not anti- social behavior. The fact that Clarisse embraces a different value set as well as approach to what she likes is in direct opposition to the social order in which they are immersed. To this entity, Clarisse is a threat, as evidenced with her questioning Guy as to whether he is actually happy. Since she poses a threat to the social order that seeks to homogenize and totalize everything, she has to be neutralized. We can only presume that this means the disappearance of her family and her being hit by a car is a part of this, but the true end is silencing her with the label of "anti- social."
To me, this is because she really likes people and she likes to think. While these are things that would be seen as good in our society, the society she lives in does not value these things. In fact, those are things that are bad in that society.
To this society, people should not think. Thinking is likely to make them want to rebel against the society. Instead, they are supposed to go in for mindless entertainment. Because Clarisse likes to think, she is antisocial.
To this society, people should not really care about each other. If people care about each other, they will tend to separate themselves into groups -- there will be the people you care about and those you don't. This might make the society split apart and come into conflict. Therefore, it is important that people do not care about each other. Since Clarisse does care about others, she is anti-social.
Overall, then, Clarisse is antisocial because she acts in these ways -- ways that would undermine the society if everyone copied her.