Yes, but they should be blanched before freezing to kill bacteria. Clean the leeks and then slice them. Bring a pot of water to a boil and then quickly blanch them (submerge in boiling water for 30-40 seconds). Let them drip dry in a colander to remove most of the water. To keep the leeks from freezing together, lay the slices on a sheet of parchment paper on top of a cookie sheet and then freeze. Leeks can also be frozen in small clumps the same way. Once frozen, the sliced leeks can be combined in a freezer safe container and enjoyed at any time.