One of the motives of mass murder is the desire for attention. Every time there is a mass murder, people give the murderer the desired attention. People wants the information on mass murderers and the media earns rating by giving such information.
We cannot change what people are interested in but we can change what the media can cover.
The incentive of fame can be completely eliminated by devising a policy to limit what the media can reveal about the mass murderer.
Make a policy to prevent the media from revealing mass murderers name and likeness to eliminate the incentive of attention.
References:
http://www.alexteveschallenge.com/
http://youtu.be/PezlFNTGWv4
http://www.policymic.com/articles/20866/adam-lanza-continued-media-cover...