The second person singular pronoun yu and the second person plural pronoun unu can be used in sentence-initial position for extra emphasis on the addressee(s), or as a politeness marker. The basic prohibitive clause is not obligatorily marked for singular or plural person reference (Van den Berg 2007: 257).
Source: Schumann 1783: 75
Source: CR 1707
Source: Herlein 1718: 123