There's no way to stop one unless we see it comming well in advance. More money should be spent on finding them. However, the only hope for the long term survival of our species is to spread ourselves around the solar system. I'd like to see the first colony on Mars be under the American flag, not some Euro-wanker's version of internationalism. But, given the lack of interest in the US space program, we'll never see it.
"More money should be spent on finding them."
Now use the terrorism model! More funding will be appropriated, right after we get struck by one.