There is no single control but try dicamba. It may take several applications with this herbicide before the onions die. You can also try a herbicide with 2,4-D & MCPP and dicamba pre-mixed. You need to buy this premixed because they will separate if you try mixing your own. Commercially they are put together while under very high pressure. Be very careful treating near/under trees and shrubs because the herbicides will damage or kill the woody plants sooner tan the wild onions.
