I use a small amount of the herb Black Cohosh which seems to help (you should not take >40 mg per day). I also use small amounts of licorice root extract (250 mg per day).
But then you can get into the problem of estrogen dominance which can stimulate the growth of breast cancer. As far as I know, only the herbs bloodroot and mandrake have been shown to both block progesterone and suppress estrogen receptor positive breast cancer.
Since progesterone is produced in large amounts by the ovaries, sometimes a complete hysterectomy or ovarianectomy can give some relief, but the adrenals can still produce progesterone. And we definitely need it, but not in large enough amounts to inhibit the chloride channel function. During pregnancy progesterone is produced by the placenta, which is why myotonia often improves for women who were miserable during their pregnancy shortly after they give birth.
I forgot to mention that fat cells also produce progesterone, so the less body fat you have, the lower your progesterone should be. A good reason for those of us with MC to keep our weight down :-)