Here's an approach with the multiaudience package.
By commenting the %\def\CurrentAudience you control which parts are compiled. The - (minus) in \begin{shownto}{-,myNotATheorem} ivnerts the argument(s).
According to the manual you can also use this to define different versions in batch files, like pdflatex "\def\CurrentAudience{SomeAudienceTag}\input{file}".
\documentclass[preview,border=2mm]{standalone}
\newtheorem{theorem}{Theorem}
\newtheorem{lemma}{Lemma}
%% The important package here
\usepackage{multiaudience}
% Define the possible audiences
\SetNewAudience{myNotATheorem}
\SetNewAudience{SomeThingElse}
%% Comment / Uncomment at free will
%\def\CurrentAudience{myNotATheorem}
\def\CurrentAudience{SomeThingElse}
\begin{document}
\texttt{\textbackslash CurrentAudience:} \textbf{\CurrentAudience}
\begin{theorem}
This is theorem 1.
\end{theorem}
\begin{shownto}{-,myNotATheorem}
\begin{lemma}
This is lemma 2.
\end{lemma}
\end{shownto}
\begin{theorem}
This is theorem 3.
\end{theorem}
\end{document}

