As neither Mr Sanchez nor Mr Loughner could have accomplished quite as much if they'd been wielding, say, lawn furniture, how do we keep guns out of the hands of such people?

What if he had used:
A knife?
A ball point pen?
A baseball bat?
A car?

A gun is a tool, just like a baseball bat, a kitchen knife or a car. Tools are only harmful if people use the tool in a harmful way.

And also anecdotal - I lived in a country where all the law abiding citizens had their guns confiscated - and all that happened was violent criminals got more violent and more people got shot because, surprisingly, criminals don;t care if they are breaking the law by having a gun.