I'm talking more about filtering on the message content than filtering based on account settings. For example, if a message is 90% the same as the previous 3 messages the user sent just silently redirect it to /dev/null. Users would quickly learn that a cut'n'paste approach gets rapidly diminishing returns. That alone would slow down a lot of the men who just spam messages to every woman on the site, improving the experience for them while leaving the genuine men on the site unaffected. But I'm sure OKCupid could do much cleverer things.
Users would quickly adapt. Something simple like spinning syntax would address that. I agree, OKCupid could be more clever, and build systems much harder to get around.
That said, I don't think the incentive is there for them to do it. Even if they removed 100% of all spammy behavior, I don't think it would solve the problem of message volume. If we assume that skimming a profile and writing a personalized message takes five minutes, a "genuine man" user is able to churn out a dozen messages an hour. Women would get less than the hundreds of messages they get today, but still far too many to work with.
From what I've seen, there's a basic contradiction at the heart of online dating. In general, women only want to be contacted by men who they are attracted to and interested in. Also in general, men live in fear of being ruled out and missing their chance with a woman they find interesting.
Here are the choices for message filtering (they updated it over the past year or so): gender, age range, whether they are located nearby, whether they are single, match %
Also, if the user pays for A-list they can filter on: how long the messages are, hide if it contains certain words, filter by attractiveness.
