Toxic plant ?
This is a type of Euphorbia, possibility Euphorbia amygdaloides, which is used as an ornamental ground-cover plant. It is poisonous if ingested. Broken stems will produce a milky sap that may cause skin irritation in some people. Wear protective gloves and use a shovel to dig out the unwanted plants and their roots.
Here is additional information about Euphorbia. http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=268086&isprof...