I agree that the cards are a pretty ineffective way of dealing with the problem at events. They're only effective for people who don't know they're doing something inappropriate, but do care when told. I think that's a very small percentage of offensive people.

Selfishly, I care more about getting the (admittedly very small number of) hacker females to attend and contribute to events more than unrestrained free speech by attendees. If kicking out a few obnoxious drunken guys, and convincing others to behave reasonably, is necessary, fine. That seems a lot more effective than cards handed out by attendees. Naming and shaming people who act like idiots would also be effective I suppose, but probably more potentially open to drama and consequences than quietly ejecting them for the rest of the event.