Since you are loading the caption package, you could use \captiosetup{list=no}:
\documentclass{memoir}
\usepackage[english]{babel}
\usepackage[demo]{graphicx}
\usepackage{caption}%This is necessary in order to ignore lof
\begin{document}
\listoffigures\clearpage
\begin{figure}
\captionsetup{list=no}
\begin{sidecaption}{Main Caption}[fig:label]%does not appear in lof
\includegraphics[width=0.5\textwidth]{Picture}
\end{sidecaption}
\end{figure}
\begin{figure}
\centering
\includegraphics[width=0.5\linewidth]{Picture}
\caption[]{Main caption} %does not appear in lof
\end{figure}
\end{document}
But using caption with the memoir document class produces a warning:
Class memoir Warning: You are using the caption package with the memoir class. This may cause unexpected or inconsistent results if you use any of memoir's captioning facilities.
This warning indicates that since caption package is being used, memoir's own caption configuration interface is not doing anything. If you want to keep memoir's captioning facilities (without the caption package), I'd suggest another approach, simply locally changing the file extension used for the figure(s):
\documentclass{memoir}
\usepackage[english]{babel}
\usepackage[demo]{graphicx}
\makeatletter
\newcommand\nocaptioninlist{\renewcommand\ext@figure{ll}}
\makeatother
\begin{document}
\listoffigures\clearpage
\begin{figure}
\nocaptioninlist
\begin{sidecaption}{Main Caption}[fig:label]%does not appear in lof
\includegraphics[width=0.5\textwidth]{Picture}
\end{sidecaption}
\end{figure}
\begin{figure}
\centering
\nocaptioninlist
\includegraphics[width=0.5\linewidth]{Picture}
\caption{Main caption} %does not appear in lof
\end{figure}
\end{document}
The demo option for graphicx simply replaces actual figures with black rectangles; do not use that option in your actual document.
captionpackage is used, thenmemoirsown caption configuration interface is not doing anything – daleif Dec 17 '12 at 14:37\makeatletter \newcommand\nocaptioninlist{\renewcommand\ext@figure{ll}\renewcommand\ext@table{ll}} \makeatotherand now\nocaptioninlistwill also suppress entries for selected table captions. – Gonzalo Medina Dec 17 '12 at 14:58.llfile (which I initially overlooked), the file simply doesn't exist since nothing is creating such file. – Gonzalo Medina Dec 17 '12 at 15:02