At last to my question! Will they reuse the nest this year or should I clean it out so they can build from scratch?

As a nature photographer, your plight has offered me so much excitement i scarcely know where to begin.

I sympathize with you at the lack of birds, i don't know how i would get through my winter days if it weren't for all the birds at my feeders but before we address how to run that hawk off, I would strongly encourage you to grab your camera and get as many photos as you can. It is a rare opportunity to see a hawk close up.

I was especially excited when you mentioned the white weasel. I suspect what you saw was a STOAT (Mustela Erminea) commonly called a "Short tailed Weasel. The Stoat ranges throughout the northern temperate and arctic range, although they are generally nocturnal and extremely elusive. In summer they are brown and called stoats, but in winter they have a white coat and they are properly called Ermine. In the fur industry the white pelt of an Ermine is prized higher than mink because unlike Mink, the Ermine cannot be raised in captivity. They are extremely difficult to trap because they will literally chew their own foot off to escape a leg hold trap. Just getting a fleeting glimpse of an Ermine is a once in a lifetime event.

Now let us consider how you might deter the hawk. About the only natural enemy of the hawk is a large owl.
they make a large owl decoy that is primarily used by gardeners to deter birds from their gardens. Erecting such a decoy where the hawk likes to perch would probably deter the hawk, but on the other hand, it may also deter the smaller birds in the same way the hawk is doing now.

I can think of one other method that may work. Most marine stores or boatyard stores stock an inflatable snake which is about 6ft long when inflated. They principally place them on boats to deter birds from roosting in the rigging and making a mess on the deck or superstructures. I don't know how effective it would be in winter, but considering that the inflatable snakes are rather cheap, it would certainly be worth trying. you can also find those inflatable snakes listed in garden catalogs, or they may have them in the garden shop at your local big box store.