Through a combination of intensive angling, netting and traps. Only using one method generally doesn't control them well enough. Crappies are boom or bust spawners. Some years they dud and produce no young, especially in smaller ponds. Then you have a boom year where they produce far too many young. That year class will dominate for years to come. If not controlled it leads to many thousands of 4" paper thin crappies. White crappies are even more prone to this than black.