Lilies need their foliage to feed their bulbs in order to grow well and bloom the following year. If you cut the blossoms, leave at least 1/3 of the stem, otherwise cut just below the blossoms to leave as much foliage as possible.

If you cut the stem all the way down, they may be ok for the following year, again maybe not. But they sure cannot tolerate the same treatment year after year.