How to evict the woodchuck and other unidentified garden plunderers.

This is my first time with a veggie garden in the ground (as opposed to a container garden on the porch). A woodchuck has deemed it his personal cafeteria, and he’s destroying everything. Re-doing the fencing would be a discouragingly big effort, ripping down growth on the outside to create space to work in, buying more chicken wire and burying it deeper than the current wire. Mr. Woodchuck keeps digging in under the fence in new places or getting through a gap I couldn’t close.
My boyfriend has been sitting out there with a bow and arrow in the evenings.

Would sprinkling mothballs around the perimeter harm the cat and dog? Would mothballs seep into the soil and harm the plants? I understand they are toxic.
How effective is sprinkling cayenne pepper on/around the plants? What about broken eggshells and noisy plastic? Chimes?

Also, something is beheading my 8 foot sunflowers just as they start to create a flower bud. Heartbreaking! My guess is it might be the large, mean black ants that hang out on the leaves, but there aren’t very many ants. It looks like something actually chewed through the stalk just under the flower, and I’m finding the unbloomed heads on the ground like some sort of threat.