べからず・まじき: the difference
べからずN1
Meaning
Do not, Must not, Should not, Cannot
Past papers
Example
空気は生きるに欠くべからざるものだ。
Air is indispensable to life.
まじきN1
Meaning
Unbecoming of, Unworthy of, Not proper for, Should not, Must not, Cannot
Past papers
Tested once in past papers
Example
リヒャルトは王としてあるまじき汚名を着せられ、やがて廃位された。
Richard was tarred with the stigma unbecoming of the position of the king and was eventually dethroned.
How to tell them apart
「べからず」: a prohibition aimed at the general public in notices and rules; use 「べからざる」 before nouns.
「まじき」: aimed at a particular status or job, 'what someone in that role must never do', usually 「〜にあるまじき態度」.
Sentence-final 「な」: a blunt spoken ban, as in 「行くな」; rough, not for formal use.