What is the ruling on a group of people agreeing to abandon a Sunnah, a permissible act (mubah), or a virtue, and preventing the villagers from performing it, and punishing those who do so? And what is the ruling on the niqab and the beard in this context? And what is the obligatory stance a Muslim must take?

The villagers have no right to prevent people from what Allah has legislated, whether it is an obligation or a Sunnah (definition), such as growing the beard and covering the face. Preventing and punishing them is an injustice and a prohibition. The way to deal with them varies: If the state does not deter them, then a Muslim must migrate from the village to a place where he can practice his religion. If the state fulfills its duty, then the matter should be referred to it to curb their evil.