There is a good chance that Laughner was a prohibited person. In the recent past he was an unlawful user of marijuana; it is possible (probable) that he is still a user. There's also some evidence that he was abusing alcohol.
We also know that he was thrown out of college pending a mental health evaluation. Given that he still lives with his parents, this means that I'd hope his parents knew he was dealing with some life challenges. He also planned the entire murder from his parent's home; police found documents and other materials in the house. I'd hope that they had some awareness that something odd was going on.
So what could have been done?
First, a friend or family member could have called NICS and had him added pending a mental health evaluation:
In addition to local, state, tribal, and federal agencies voluntarily contributing information to the NICS Index, the NICS Section receives telephone calls from mental health institutions, psychiatrists, police departments, and family members requesting placement of individuals into the NICS Index. Frequently, these are emergency situations and require immediate attention. Any documentation justifying a valid entry into the NICS Index must be available to the originating agencies.I just learned about that option myself. That may have stopped him from purchasing a firearm from an FFL, although he could have always gotten one from his drug dealer. Still, it might have helped.
Once he'd acquired the handgun, he took videos of himself and posted them on YouTube. Why didn't one of his MySpace "friends" (some of whom seem to also have known him in real life) or his parents intervene? I mean, back in December, Laughner posted about wanting to kill police officers. People knew he had mental health issues and had substance abuse problems. If one of your friends was dealing with similar challenges, ranting about killing police officers, and you knew they had just purchased a handgun and were in illegal possession of a firearm, what would you do?
If police had been brought in at this point, then the video evidence of a firearm combined with testimony about substance abuse would probably be sufficient for a search warrant. One joint combined with the firearm would have sufficed to file federal charges and likely state weapons misconduct charges. That would have gotten Laughner off the street and into the system. He could have been put in jail or the DA could plead him out on condition of getting mental health assistance. Either way he'd have been unable to carry out his plot.
Unfortunately, it seems like his home situation might have been a bit odd. Maybe the parents were not fully engaged or were part of the problem. And one well known problem with groups is that the larger the group, the less likely it is that any individual will intervene or take action because they think someone else already has.