莫 + [verb]
莫 is a negative from classical Chinese that survives in signs, sayings and fixed words. It has two meanings: "do not", and "no one" or "nothing".
In signs and advice, 莫 means 不要 or 别. It is short and old-fashioned, which suits a notice or a saying.
In fixed words 莫 means "no one" or "nothing": 莫名其妙 "that no one can explain", 爱莫能助 "willing but unable to help", 莫大 "than which nothing is greater".
莫不 "everyone without exception", 莫非 "could it be that" and 莫过于 "nothing is more … than" have their own entries.
莫 doesn't replace 不 or 别 in conversation. Outside sayings and signs, it sounds like a line from an old novel.