Dig poison ivy out
Dig mature plants in late fall or early winter when the foliage has dropped off. Prune back the stems to about 6 inches above the soil level. Then with a mattock or grub hoe “grub out” the roots down 4 to 6 inches under the soil. Because poison ivy roots can spread as far as 20 feet, you may miss some. Search out and dig up root sprouts and seedlings from seeds the birds have dropped in the spring.
Prune it to death
In the spring when poison ivy is just coming into leaf after the winter cut the stems right back to soil level. In a week or two, come back and do it again. The secret is persistence. Mature root systems keep sending up new shoots for as long as two months. Eventually, however, the plant will die from this treatment. Bag cut stems and discard them in the trash.