Virginia Tech

Good God.
Thirty-two young lives ended way before their time.
This isn't tragic. It's disgusting. It's evil.
I have to admit, I'm a little sick of having lived through so many "historic" events like this.
How do you stop a person bent on murder who is willing to die to achieve his ends?
You can't.
You can talk all day long about fences, checkpoints, lockdown and the like, but at the end of the day, if someone is crazy enough, determined enough and smart enough, there's going to be bloodshed.
You can legislate against guns until you're blue in the face, but a determined killer will still find a way.
I'm sorry, but as far as guns go, the genie's out of the bottle. Has been for oh, seven-plus centuries now. You could ban them today, but there will still be guns. If convicts in a controlled environment like prison can make them (and they can), anyone in society with a modicum of mechanical know-how can. They're not going to go away, no matter how much some of us would like.
You cannot prevent things like this from happening.
The only thing you can do is mitigate them, limit the damage.
You do this by killing the perpetrator as soon as possible.
Why do so many of these massacres take place in institutions of learning? There are several reasons:
There is a concentration of potential victims, presenting to the murderer what the military would refer to as a "target-rich environment".
Campus police forces tend to be tiny in relation to the populations they protect, and ill-equipped to handle such events. I have yet to hear of a university SWAT team.
Many (if not all) schools like this have a zero-tolerance policy towards firearms, ensuring that the murderer will be able to have his way with relative impunity and safety.
The perfect storm; large groups of densely-packed defenseless people with very little in the way of police protection.
If the authorities cannot guarantee their safety (and they obviously cannot, as this and other such events show us), then for the love of God allow the students the means and opportunity to protect themselves.
Firearms-free campuses are a disaster waiting to happen. They obviously don't work.
Arm the students.