Remove comma from moderncv's \cventry

The entry you're showing is produce with \cventry, so we want to change the definition of this macro. It is defined in moderncvstyleclassic.sty. All I did is copy it into my preamble verbatim and remove the undesired comma. (In the MWE, I actually copied the line and commented out the original, to make the change recognizable.

\documentclass{moderncv}
\moderncvstyle{casual}
%%%
% the following definition is from the file moderncvstyleclassic.sty
\renewcommand*{\cventry}[7][.25em]{%
  \cvitem[#1]{#2}{%
    {\bfseries#3}%
%   \ifthenelse{\equal{#4}{}}{}{, {\slshape#4}}% I changed this line (with comma) ...
    \ifthenelse{\equal{#4}{}}{}{ {\slshape#4}}% ... into this one (without comma).
    \ifthenelse{\equal{#5}{}}{}{, #5}%
    \ifthenelse{\equal{#6}{}}{}{, #6}%
    .\strut%
    \ifx&#7&%
      \else{\newline{}\begin{minipage}[t]{\linewidth}\small#7\end{minipage}}\fi}}
%%%
\firstname{John}
\familyname{Doe}
\begin{document}
\section{Education}
\cventry{December 2012}{Master of Science in Chocolatology}{University of Candyland}{Sugartown}{A+ with Golden Gummy Bear}{I am the chocolate-man.} 
\end{document}

instead of redefining the whole command yourself also consider using xpatch:

\usepackage{xpatch}

\xpatchcmd\cventry{,}{}{}{}

this line replaces the first occurence in the macro text of , by the empty string - i.e. removes it.

Here is also a nice documentation of how the command works.

Obvious comment, but in case you want to be able to switch between having and not having the comma, you can define a new command \cventrynocomma like this:

\newcommand*{\cventrynocomma}[7][.25em]{%
  \cvitem[#1]{#2}{%
    {\bfseries#3}%
    \ifthenelse{\equal{#4}{}}{}{ {\slshape#4}}%
    \ifthenelse{\equal{#5}{}}{}{ #5}%
    \ifthenelse{\equal{#6}{}}{}{ #6}%
    \strut%
    \ifx&#7&%
      \else{\newline{}\begin{minipage}[t]{\linewidth}\small#7\end{minipage}}\fi}}