What is the usage of 蔑 as a negation word in classical chinese?
Here's a good English definition and explanation with an example sentence to get you started:
A Students Dictionary of Classical and Medieval Chinese

2 negative particle of the perfective: had not, have not, will have not; similar to 末 mò and often accompanied by perfective-final 矣 yǐ; mostly found in Zuozhuan 左傳 and Guoyu 國語, later mainly for archaic effect; e.g. 寧事齊楚, 有亡而已, 蔑從晉矣 níngshì Qí Chǔ, yǒu wáng éryǐ, miècóng Jìn yǐ, Better to serve Qi and Qi, lest we simply perish, and [better] not to have followed Jin.
3 not to have, lack; there are no…

Next you can have a look at these dictionary samples to get a better grip of how it works (some of the definitions are the same, but they use different sources):
MoE

◆ &lt;副&gt; 無、沒有。
如：「蔑以復加」、「蔑不有成」。

Liang'An

2 〈書〉無；沒有。
例：～以復加∣～不有成。

Hanyu Da Cidian

4 无；没有。
《诗·大雅·板》
喪亂蔑資, 曾莫惠我師。
毛 传
蔑, 無。資, 財也。
《史记·孔子世家》
夫子循循然善誘人, 雖欲從之, 蔑由也已。
《新唐书·突厥传赞》
自《詩》《書》以來, 伐暴取亂, 蔑如帝神且速也。
明 胡应麟 《少室山房笔丛·华阳博议上》
即小學一端, 門逕十數, 古今博洽, 蔑能相兼。

Longmans

❷ 沒有﹐無：蔑以復加（沒有甚麼能再增加的了）。

Gu Hanyu Cidian

④无。《左传•僖公十年》:“蔑不济矣。”