Gradually. And with patience. Artemisia vulgaris and a few other species are notoriously hard to eradicate if they are growing in ideal conditions. The best approach is to dig up as much of the root ball as you can, then keep an eye out for volunteers that spring up from bits of root or from seeds that have fallen in the soil. If digging up volunteers proves too time-consuming, a careful spot treatment with a broad-spectrum herbicide such as glyphosate will kill the plants—but make sure not to allow overspray to contact other plants. Be prepared to deal with stubborn residual volunteers for quite a long time.