You ask how to temporarily turn off a global option.
In general you have to look at the code (in the class or package) that sets the global option, create a macro that negates that code and also a macro that reinstates the option code. Something along the lines:
\documentclass[opta]{...}
\newcommand{\killopta}{...}
\newcommand{\enableopta}{...}
%
\begin{document}
bla bla bla
\killopta % disable option opta
more blas
\enableopta % reenable option opta
yet more blas
\end{document}
ADDITION
In a comment the OP asked how to deal the the nomath option in the fontspec package.
I have never used fontspec but looking at the fonspec code as given in fontspec-luatex.sty there is the following code:
\DeclareOption {math} {\bool_gset_true:N \g__fontspec_math_bool }
\DeclareOption {no-math} { \bool_gset_false:N \g__fontspec_math_bool }
I would guess that creating two new commands, say \mathon and \mathoff whose definitions are the same as for the math and no-math options would enable you to switch between them in the document.
\ensuremathis wrong. – Henri Menke Jan 07 '20 at 01:18ensuremath. What is the correct use of it? – Niranjan Jan 07 '20 at 08:47$...$then removed the$. See also https://tex.stackexchange.com/a/35877 – Henri Menke Jan 07 '20 at 09:08