Plant in a new area and protect the plants with row covers, secured around the planting so the they adult flies can't get near the plants.

Of course, that depends upon how many plants you have. (You didn't tell us.)

What is your soil like? There are beneficial nematodes that will aid in controlling root maggots, but they will only live in soils that are well amended with organic matter. I have seen people, with very low levels of organic matter in their soil, that have problems growing any root crop because the root maggots and wireworms decimate those crops, but I also know people with adequate levels of organic matter in their soil (in the same neighborhood) that do not have these problems.
You can spend a lot of money on poisons, Diazanon for expample, that will in addition to the root maggots will kill off the beneficial insects that would eat those root maggots, or make your soil into a balanced eco system that will control this for you.

Sorry, but Diazinon isn't available for home use.

That said, home gardeners with some still in their stash can use it according to label directions.

As for nematodes, they're an iffy proposition in the garden.