Kill all minecarts on a specific block
I found it. At https://gist.github.com/Dinnerbone/3736487, there is information stating that to target entities at a block, you use:
@e[4,4,4,r=your_radius,type=MinecartRideable,tag=value...]
where 4,4,4 are your x y and z, and your_radius is your radius.