Pull it up and dig out any roots.

Horehound is in the mint family. It grows much like the horrible Bindweed aka, Morning Glory. Very aggressive roots make their way across large areas in just one season. My neighbor accidentally introduced mint to the neighborhood decades ago. I fought it by digging and pulling and pulling and chopping for years, and though it cleaned up the beds appearance, disturbing those roots that way was the worst thing I could have done, it encouraged more mint. Your horehound grows the same way. I resorted to Roundup. Really soaking it and doing it every 14 days. Don't pull them after a few days. Let the weed get good and dead. I sympathise and wish you luck. Here is a link:http://www.gardeningknowhow.com/edible/herbs/mint/invasive-mint-how-to-kill-mint-plants.htm