There is no such thing as the perfect response to street harassment. The reason for this statement stems from the wide variety of behaviors exhibited by street harassers combined with the different circumstances of each incident of harassment all compounded by the different feelings and attitudes of the targets of harassment.
The suggested response of “Not interested!” is nothing other than a tool to facilitate assertive communication. And just like every tool, it is not suitable for every situation, but it is suitable for most situations of harassment. “Not interested!” is like a hammer. You can swing it lightly and make it a tap hammer, or you can swing it with all your might and make it a sledge hammer. Therefore, it provides a great deal of versatility.
A loud and powerful “Not interested!” serves the same function as “Back Off!” Remember, it is not what you say, it is how you say it that conveys the message. But sometimes a hammer will just not do. The nail is bent and you need a pair pliers to pull it out.
“Not interested!” works so well because the majority of the time your emotional state is one of repulsion from fear and apprehension. But there are instances where you are in fact “Interested!” You are interested in conveying your outrage at what has happened. You are interested in stopping the behavior. You are interested in seeing that the harasser is punished. This emotional state is not reflected by the “Not interested!” statement.
One immediate situation that comes to mind is public masturbation. If you have learned verbal assertiveness, you might want to use a strong behavior commands such as “Stop that now!” Use the same voice you use when your dog does something disgusting in front of your family or friends. You could also choose a more indirect approach and take a picture as advocated by Hollaback. You could do both. In either case, you have taken a personal interest in dealing with the behavior.
Unwanted touching and groping is another situation where “Not interested!” may not be enough for you. Again, the same tools are available as previously mentioned to deal with this type of behavior.
If the situation becomes more threatening and potentially violent, it is no longer street harassment. It is assault. This higher level of physical danger requires you to be “interested!” You must be so interested in stopping the aggressor that you are able to cause him to disengage and cease his aggression.
Break the cycle of deny, delay, and do nothing.
Tell them that you are "Not Interested!" and move on.