Over the last couple of years, a few conference organizers have told us that their conference does have a policy for attendee behavior - it's just not public! In our experience with dozens of conferences, this means their conference is losing most of the benefit of having an anti-harassment policy at all.
Anti-harassment policies aren't about punishment after the fact, they are about preventing harassment in the first place. They educate everyone in your community, set the tone of the conference, and make it far more likely you'll find out about harassment that does happen. They increase the variety and diversity of people at your conferences and widen the pool of talent to draw speakers from. But if your policy is private, the only effect it can have is after the harassment has already happened.
Here's why your conference should have public, well-promoted policy with specific guidelines for attendees:
You'll have more people at your conference: When people are choosing which conferences they attend, they look for an anti-harassment policy as a sign of what the whole conference will be like. It not only answers the question, "Will I be safe here?" but also "What will the overall atmosphere be like?", "How good will the talks be?", and "Are the conference organizers focused on the attendees having a good time?" It also tells them that the other attendees have at least some idea of what appropriate behavior is. People can only go to so many conferences a year, and if your conference is in competition with another conference that does have a public policy, you'll lose attendees to it. This is especially true for women and in general people who will increase the diversity (and the value) of your conference.
You'll have less harassment at your conference: Often people don't even know that they are doing something that harasses other attendees. Occasionally they know it is harassing but assume that "that's just how conferences are" and it's okay for them to behave that way at your conference. It may be inconceivable to you that people don't know to not include pornography in a technical presentation, or not to touch people sexually without permission, but this list shows that plenty of people do not know that. And the larger and more successful your conference, the more likely someone will show up who doesn't agree with your ideas. Help your attendees succeed: give them the information they need to have a great conference.
You are more likely to hear about harassment when it happens: A lot of people have had bad experiences with reporting harassment. In the worst case, they get harassed a second time by the person they report it to! This has happened at conferences, especially with untrained volunteer security staff (who sometimes kick off the harassment themselves). Most attendees assume that conference staff won't do anything about an attendee being obnoxious unless the conference staff tells them in advance that they are eager to hear about and help in these situations. Being very specific about what you consider harassment helps a lot: A vague policy leaves open the question about what the organizers consider harassment. Reporting harassment is already extremely hard for people to do; give them all the help you can. The alternative is that you won't learn about harassment, and one or more of the following will happen: The harasser may harass more people at your conference, the victim may not come back to the conference, and the incident might become public months or years after the fact, causing many people to stop coming to your conference.
Harassment is more likely to be stopped immediately: A public anti-harassment policy can end an incident of harassment immediately. The victim or a bystander can point out that the harasser is violating the official conference policy, which results in the offender stopping and/or apologizing immediately. Without a policy, it is more likely to escalate into an argument about, e.g., whether commenting on a woman's breasts is just a compliment. The great thing about situations in which the policy resolves the situation is that your conference staff doesn't even have to get involved. Less work for everyone!
The bottom line: People use a published policy to judge whether to attend a conference, whether to report harassment, whether to engage in harassing behavior themselves, and whether they can safely challenge harassing behavior. For conference organizers, a published policy is a tool to improve their conferences' image, increase attendance, reduce the chance of harassment, and increase the likelihood they will hear about harassment. None of these can occur if the attendees don't know the policy exists, or if it is too vague to understand what the organizers expect.