Caryopteris also known as blue mist spirea (Caryopteris x clandonensis) is rated hardy in zone 6 to 9 by some experts but mine, like yours is thriving in zone 5 and I have seen them perform well in parts of zone 4. Healthy vigorous plants will reseed readily. I invite friends to dig seedlings for their yards and then pull whatever is left. You can spot treat the seedlings with Roundup, Finale or another total vegetation killer. Just be careful not to hit nearby desirable plants as these products can also kill them. The best time of the year to prune them is late winter or early spring before growth begins. Prune the plants back to 4 to 6 inches above the ground. As the plants age you may decide to cut the plant back to 12 inches. Then remove some of the older stems back to an adjoining branch.