Once a Lag B'Omer celebration was being held in Jerusalem. Several yeshivah students from overseas joined the gathering. They had grown beards during the Counting of the Omer, but had removed them on Lag B'Omer. The Gaon, Rabbi Yeshaya Osher Zelig Margolis of Jerusalem, told them: "Throughout the days of the Omer, you restrained yourselves from removing your beards. Today, on the day of Rabbi Shimon's celebration, you too are joining in the rejoicing. But how are you are marking his holy day? By trampling on his words. This recalls the words of the Megilah: 'Would you assault the queen while I'm at home?!'" My words bore fruit and the students resolved never to touch their beards.
Rabbi Shimon bar Yochai taught (Zohar, Vol. III, p. 130b): "Woe to those who extend their hands against the precious, sublime beard."
The Chafetz Chayim writes: "Removing one's beard (even with scissors) brings about destruction and blemishes [in the spiritual worlds] Above and removes the image of G-d [Tzelem Elokim] from one's face. By doing so, one uproots the channels of holiness from Above... as explained in the holy Zohar and the works of the Kabbalists.... Accordingly, any person with a sensitive heart should be in dread of this severe and bitter sin. If he transgressed, he should hurry to repent and should not pay attention to his foolish friends whose evil inclinations have overpowered their minds" (The Chofetz Chaim in his Kuntres Tiferes Adam, Introduction, sec. 1; ch. 10).