They look like aphids from what I can see. I usually spray them with a bit of insecticidal soap, but obviously that's not a good idea in the aquarium. Continue to manually destroy them, and if you can't get them under control that way, use a peroxide dip. Or you could just submerge the plants; the little guys can't survive underwater, and will soon starve without a plant to feed on.
I think you were correct on your Ludwigia repens ID. It looks a bit different in the wild than it does when tank-raised.