A friend pointed us at a very good article about how to properly enforce anti-harassment policies once they are in place. This is particularly important for any Backup people who are not part of the con staff.
Unless the harassment is overtly threatening such that it is obvious from a reasonable distance (such as the incident that was the genesis for the Backup Ribbon Project), it is best to make a statement along the lines of “hey, that is not cool.” If the person says “Oops, I am sorry,” then let it go. If they try to deflect or turn on you, or it is a repeat offense, then *only after* you have made sure the person being harassed is safe, make a report to con com, so that they have it on record.
Obviously, the first priority is the safety of the person being harassed. However, once that has been established, it then becomes important to start a documentation trail with the con com so that it is on record that they know about the incident in question. This not only can help the con com to make the event a safer space for everyone, but also highlights that the point of the Backup Ribbon Project is not to be lone vigilantes a la Batman (although, we would be lying if we said we did not want his utility belt), but to work with the con com to the benefit of all attendees.