A species of commelina aka dayflower

Hand pulling is the only way, but you have to be diligent. Any little section that doesn't come out will sprout another plant, plus, they come up from seed. It's just one of those things that is really hard to get rid of, but in the meantime, you can put the tips and flowers into salads to keep it from going to seed. Free food!

Are those the same blue ones that grow up north?

There are a number of varieties/variations of this plant, I believe. It can make a groundcover all by itself.