Rav Herschel Schachter (quoted in the OU's Jewish Action magazine) seems to indicate that yes, a Rabbi/Dayan's rulings can be annulled:
Jewish Action: Can one still follow the piskei halachah of a fallen rabbi?
Rabbi Schachter: No. The pasuk in Navi (Malachi 2:7), as expounded by the gemara (Moed Katan 17a), says that a Torah teacher must be sinless and righteous like a malach (angel). According to the Torah, we only follow a rabbi’s ruling if he properly models Torah behavior. If he is a ba’al aveirah, if he knowingly violates Biblical or rabbinic laws, he is not qualified to teach and render halachic rulings. When members of the public become aware of his improper behavior, they may no longer rely on his judgment for any rulings, unless it can be verified that such rulings were rendered before the rabbi’s sinful conduct began. Since it is often not possible to ascertain when these rulings were rendered, one should ask another rabbi for a new pesak.
While Rav Schachter does say that if we can verify the rulings took place BEFORE the improper behavior we can rely on said person's ruling, this is pretty much impossible and there's no way to know when said sinful conduct began.
NOTE: This is referring to a singular person, and not a group where a Dayan was part of (ie a Beis Din).
Worth checking out the article for more details.