IMO, once the larvae are inside the leaves, there isn't much you can do besides remove the infected leaves. The time to take preventative action is when the adults are present and laying their eggs on the plants. The adult flies are present at different times in different parts of the country, so you have to watch for them.
When the flies are present, you can spray with Neem oil or a soap solution. At that time, you can also put out sticky traps for them. There are also tiny wasps that are parasitic on the flies. You can buy them and release them at the time the flies will be around.
There is really no way to kill the larvae while they're protected inside the leaf. Some people use their fingers to try to smash the larvae inside the leaves, but they are so small, I don't know if there's any guarantee that that works.