Remove their shade. Redworms need adequate shade to flourish. They don't do well under direct sunlight. Therefore, if you find them swarming under shade, take away the shade. Prune the plants under which they hide and remove big stones sheltering them in your garden. When exposed to direct sunlight, they will die or crawl somewhere else with shade.
Keep redworms from overrunning your garden. Gather these worms and place them in a worm bin to keep them off the garden.
Flood the area where redworms hide. These worms don't like a lot of water or dryness. Extremes of moisture and sunlight will kill a good number of them.
Introduce the redworms' natural predators. These are moles and centipedes, not birds. Most garden or farm predators are turned away by the bad smell and yellowy pus secreted by redworms.