An act is disallowed by the authority that might have allowed it; the word is especially applied to acts which are done before they are pronounced upon; thus, a government may disallow the act of its commander in the field or its admiral on the high seas.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

illegal Do you think that handguns should be made illegal?
banned The school banned the students from using Tipp-ex.
prohibited An accident prohibited his attending the ceremony.
proscribed That word is proscribed by the French Academy.
invalid From tomorrow this email address will be invalid.

Popularity over time

Word	1850 → 2019	Trend
disallowed		falling
illegal		steady
banned		rising
prohibited		steady
proscribed		falling

Use in printed books, per decade from 1850 to 2019.

By length

6 letters: banned¹
7 letters: illegal³, invalid³, illicit³
8 letters: verboten³, nonlicet, nonvalid
9 letters: forbidden³, unallowed
10 letters: prohibited⁴, proscribed², nonallowed
11 letters: interdicted⁴
12 letters: illegitimate⁵
13 letters: impermissible⁵

The small number after each word is its number of syllables (illegal³ = 3 syllables).