Q. Are you 100% sure it's millipedes causing the "real" damage? Could it instead be something like cutworms causing the "real" damage and the millipedes are just feasting on the remains?
To check if you're dealing with millepedes (which tend to be dark in colour), squash one, if there is a strong "stinky" smell after doing so, then you have millipedes.
Q. Are the plant-beds mulched? If yes, with what?
As far as I'm aware all the organic control options given here on SE should also work on millipedes. That said, I think of the 6 options given, "Diatomaceous Earth" might be the best option in your particular case. Since being reminded, told about "Diatomaceous Earth" I keep hearing, reading about how effective it can be as an organic pest control option when placed around plants...
Vacuuming might be another organic control method worth looking into (at least until you have the problem under control):
Some additional reading from the Department of Agriculture and Food (Western Australia):
Last two links above are broken. Maybe updated versions are presently at: 1
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