Oh my, Euphorbia lathryus. It's the irritating sap that is said to repel gophers. Obviously it doesn't work. It reseeds easily.....as you've noticed. Rip it out, but be VERY careful, the latex sap can burn your eyes or other soft tissues if you rub them. Consider wearing plastic bags over your hands and then throwing the bag away when you are finished pulling and handling the plants. As for the seedlings, hoe, hoe, hoe. Perhaps one of the growth inhibiting herbicides would work......Dacthal like products. But in the long run, hoeing out the seedlings is safer.