cmom, how long have you had your coop/run setup?
Volume904, mothballs are toxic to chickens.
And all who mentioned snakes, 1/2" hardware cloth and sealing up all holes to coop will keep snakes out of both coop and run. However, I have seen pics of a snake scaling a 4" run wall, so do be sure to cover tops as well ~ if you want to completely keep them out.
Chicken wire is the cheaper choice, but it is a gamble. If you do a search back through the "predator" section of this forum, you will see many, many sad stories of folks who lost chickens ~ or entire flocks ~ when predators peeled back the chicken wire and got into runs. That gamble is a personal choice, but it is still playing the odds.
The only preds I am aware of that might be able to get through a solid 2 x 4 run with well-attached hardware cloth are big cats and bears. I would welcome any information to the contrary.
Good luck to those building new runs! There are a lot of good ideas to utilize on the forum.