Welcome to the Universe Today forums John.
I heard a great solution on
Quirks and Quarks the other day which involved a giant airbag on a spacecraft which could then repeatedly bump against a rotating asteroid to nudge it into a different trajectory.
Asteroids are a huge threat to life, and the first step is definitely to seek out all the largish Earth crossing objects. I wonder if we'll ever be able to find the 100 metre objects, though. And, we're still victims to comets which can come from nowhere at high velocities.