how to disable/enable etoolbox global command effect at different locations in a document?

A possibility is to create a switch and depending whether the switch is true or false execute the \clearpage command.

\documentclass[12pt]{article}%        
\usepackage{etoolbox}
\usepackage{lipsum}

\newtoggle{clearpagebeforesection}

\preto{\section}{\iftoggle{clearpagebeforesection}{\clearpage}{}}

\begin{document}
\toggletrue{clearpagebeforesection}
\section{A}
\lipsum[75]

\section{B}
\lipsum[75]

%do not add \clearpage here, how? 
\togglefalse{clearpagebeforesection}
\section{C}
\lipsum[75]

%now start adding \clearpage, it is ok
\toggletrue{clearpagebeforesection}
\section{D}
\lipsum[75]

\section{E}
\lipsum[75]
\end{document}

One possibility is to create a command that locally (read in group) relaxes \clearpage:

% locally disable clearpage
\newcommand{\specialsection}[1]{
\begingroup
\let\clearpage\relax
\section{#1}
\endgroup
}

This way, \clearpage is set by default to all the sections in the document, but to the \specialsections it doesn't apply.

The complete example:

\documentclass[12pt]{article}%
\usepackage{etoolbox}

\preto\section{\clearpage}

% locally disable clearpage
\newcommand{\specialsection}[1]{
\begingroup
\let\clearpage\relax
\section{#1}
\endgroup
}

\usepackage{lipsum}
\begin{document}
\section{A}
\lipsum[75]

\section{B}
\lipsum[75]

% we use the new command
\specialsection{C}
\lipsum[75]

\section{D}
\lipsum[75]
\end{document}