I do not believe this has got anything to do with IT nor with sexism. If you are an attractive woman (relative to the attending male population), you are going to attract attention. That is how men are wired. That is our culture. Men have to approach women, or they don't laid.
Pure speculation, but the man who grabbed the woman's ass probably did it because he was in a mischievous mood, and to express his sexuality, and he probably needed to do something like that and get away with it to be happy. In other contexts, the victim might even have smiled at him (or punched him with a smile on her face). That might be what he needed. I grab my girlfriend's ass sometimes and it feels good to be allowed to do that.
The problem is probably that these are men that don't get out much, have low social skills, don't have experience with how to express their sexuality in an appropriate way, and things go wrong. And things go wrong in ways that make women very uncomfortable.
The problem is probably not that women are not being respected or valued. To me, sexism would be like automatically assuming a woman is not going to be able to perform some task, because she is a woman. Grabbing someone's ass is not sexism, it's inappropriate sexuality.
What can we do about it? Teach men how to express their sexuality in a way that works. Teach men that there are places women go to when they want to receive that kind of attention, teach them to recognize the signals, and teach them that IT conferences are not one of those places.
The cards are a really good idea just because it makes men aware that they are making her uncomfortable. Believe it or not, most do not realize.