If the Kuiper Belt object starts really far out, like 10,000 AU, then when it passes Pluto on the way in, it will be so close to perihelion compared to the overall size of its orbit, that its velocity will already approach 90,000 mph. Worst case scenario, it would take only 4.29 years to travel from Pluto-distance to impact Earth. Brutal.

I wonder if the best strategy in this case is simply to put something in it's way. If it hits something before it hits the Earth, it might create a gradually spreading cloud of debris, the majority of which could miss the Earth.

A super-hard shaped penetrator perhaps, just sitting there in its path, might do the trick.