Increase the amount or organic matter in the soil by working it into the top soil and using it as a mulch. Green manure is particularly effective at combating nematodes. Organic matter encourages the growth or other microorganisms that the nematodes do not like. This makes the environment less hospitable.
Plant suppressive plants in the effected area. These plants release chemicals that are toxic to the nematodes as they grow. They will not completely eradicate the nematodes but will reduce their population enough that your plants can go. Marigolds are easily found suppressive plants. They come in many varieties so will grow in almost any climate. Plant them in any empty space in the affected area for the best result.
Remove all the plants from the area and rework the soil using chemical pesticides, called nematicides. These chemicals are toxic and should be used as a last result. They come in many forms including liquids and granules, but they all need to be worked into the soil before the area is planted. Wear protective gloves when working with these chemicals, and follow the manufacturer's instructions. In the case of granules, spread them on top of the soil, and then work them in when you till the soil.