Rabbi Moshe Feinstein (Russia-New York, 1895-1986), in his Iggerot Moshe (Even Ha'ezer, 1:96), addresses a question concerning a certain composer of Jewish music who strayed from the path of proper Jewish observance, and regarding whom rumors of inappropriate conduct began to spread. This composer's songs had become popular in religious circles before his improper conduct surfaced, and the question arose whether it was permissible to continue singing his compositions after he earned a reputation for sinful behavior.
Rabbi Feinstein ruled that the songs may be sung despite the composer's departure from proper observance, and he draws proof to his ruling from a Gemara in Masechet Yoma. The Gemara records that Yochanan Kohen Gadol established the law of Demai, which requires separating tithes from produce purchased from an Am Ha'aretz (unknowledgeable individual), despite the possibility that the Am Ha'aretz had already done so. This Halacha has become forever known as the enactment of Yochanan, despite the fact that, as the Gemara tells in Masechet Berachot, Yochanan ultimately became a Tzeduki (member of the heretical Sadducee sect). Since Yochanan enacted this Halacha while he was still Torah observant, we may continue abiding by this provision and attributing it to him, even though he eventually resorted to a sinful lifestyle.
Similarly, Rabbi Feinstein rules, it is permissible to continue singing songs composed by a person who later became a sinner, since the songs were composed while he was still observant.
