From a societal standpoint, a "hard" approach demonstrably does not work. Gangs are a response to structural problems confronting inner city youths, which included institutional racism, poverty, and the utter lack of opportunities available for legal advancement. Until we solve, or at least partially remedy these issues, we will not make a dent in the gang problem in our cities. Certainly, when it comes to protecting citizens' lives, police should use a so-called "hard approach," and nobody is seriously suggesting that gang activity should be tolerated. But it will not solve the problem.
Yes, you need to do both. However I really do think a hard approach is necessary. It depends on the area. The economy is bad, cities are hurting, and there are fewer police patrols. Gangs know this, and are getting away with murder. Literally. We cannot just go soft on them. We need to find effective ways to both prevent and stop gang violence.
Our school has the local city's gang task-force come and talk with faculty and staff ever August before school starts. It is very interesting to hear the officers discuss facts from years of research that they have collected and shared with other cities across the country. The main policy in our town is to stop the little issues before they become big issues. That way, the gang members will know where the line is drawn and won't get very far without strict and swift attention. For example, they said that research shows that gangs are controlled best in towns where graffitti is taken down (painted over or cleaned up) immediately after it goes up. Graffitti is one way that gangs call each other out through tagging walls and other public places; hence, covering it up quickly limits that activity. Teachers are also taught to watch for signs and symbols written on notebooks, textbooks, or even students' arms, etc. Of course, the administration's job is to uphold clothing standards so gang clothing isn't worn at school, either. The officers always stress that it is a community working together with the police force that has the most affect on the rise of gangs in their neighborhoods.
Both approaches are surely needed to reduce gang activity.
Hard approaches are needed to reduce gang activity in the short term. The police need to crack down on and arrest drug dealers and those engaging in gang violence just to let them know that they are serious about reducing gang activity. However, we cannot simply arrest our way out of the problem. In the long run, only soft approaches can work. We need to provide youth with incentives and opportunities that will keep them from wanting to join gangs. This is the only way to truly reduce gang activity over the long term.