The problem is that the `comment` package doesn't seem to alter the `\endfoo` macro. The following does `\let` the `\endfoo` to `\empty` so that it doesn't call the `enumitem` internals which lead to the error. Also we have to restore `\foo` and `\endfoo` in `\uncommentfoo`, for this we again just use `\let` and don't let the `comment` package do this job (it will not restore the list behaviour, and then we'd get some `Lonely \item` errors).

```tex
\documentclass{article}

\usepackage{enumitem}
\usepackage{comment}
\newlist{foo}{enumerate}{2}
\setlist[foo]{label=\arabic*.}

% save the definitions of \foo as a list
\let\fooList\foo
\let\endfooList\endfoo

% clear \endfoo and set up \foo to start the comment behaviour
\def\commentfoo{\let\endfoo\empty\excludecomment{foo}}
% restore both \foo and \endfoo
\def\uncommentfoo{\let\foo\fooList\let\endfoo\endfooList}

\begin{document}
\begin{foo}
  \item This should be shown 1
\end{foo}
% works until this point and even works beyond
\commentfoo
\begin{foo}
  \item This should be hidden
\end{foo}
\uncommentfoo
\begin{foo}
  \item This should be shown 2
\end{foo}
\end{document}
```

Since the original answer has the problem that `comment` requires the environment's `\end{foo}` to be completely on its own in a line without anything preceding it and anything following it (including spaces), the following is a new answer, that should work (almost) arbitrarily.

It utilizes the `parser` module of `pgf` to gobble everything regardless of braces. It even tries to locally change the category code of `%` to ignore comments, but this will only work if it's not in an argument of another macro (so that the category change does apply). As long as you don't use `%\end{foo}` to end the environment this limitation doesn't really matter, though.

```
\documentclass{article}

\usepackage{enumitem}
\newlist{foo}{enumerate}{2}
\setlist[foo]{label=\arabic*.}

\usepackage{pgf}
\usepgfmodule{parser}

% save the definitions of \foo as a list
\let\fooList\foo
\let\endfooList\endfoo

% clear \endfoo and set up \foo to start the comment behaviour
\def\commentfoo
  {%
    \let\endfoo\empty
    \def\foo
      {%
        % set up a parser rule for \end, which grabs an argument and compares
        % that argument to the string 'foo', if they match, end the parser
        \pgfparserdef{commentfoo}{initial}\end[m]
          {%
            \def\tmpA{####1}\def\tmpB{foo}%
            \ifx\tmpA\tmpB
              \pgfparserswitch{final}%
            \fi
          }%
        % need to formally \end the environment, so do this after the parser is
        % done
        \pgfparserdeffinal{commentfoo}{\end{foo}}%
        % ignore input for which no parser rule is defined
        \pgfparserset{commentfoo/silent=true}%
        \catcode`\%=12
        \pgfparserparse{commentfoo}%
      }%
  }
% restore both \foo and \endfoo
\def\uncommentfoo{\let\foo\fooList\let\endfoo\endfooList}

\begin{document}
\begin{foo}
  \item This should be shown 1
\end{foo}
% works until this point and even works beyond.
\commentfoo
\begin{foo}\item This should be hidden\end{foo}This should be shown because it's
not inside of \texttt{foo}.
\uncommentfoo
\begin{foo}
  \item This should be shown 2
\end{foo}
\commentfoo
\begin{foo}THis}is an{ unbalanced token list that's completely #gobbled\end{foo}
\end{document}
```

Use `\iffalse`  ... `\fi`:

```
\documentclass{article}
\usepackage{enumitem}
\newlist{foo}{enumerate}{2}
\setlist[foo]{label=\arabic*.}
\let\commentfoo\iffalse 
%\let\commentfoo\iftrue % for printing
\let\uncommentfoo\fi
\begin{document}
\begin{foo}
  \item a
  \item b
\end{foo}
\commentfoo
% This should be hidden, i.e. comment out
\begin{foo}
  \item a
  \item b
\end{foo}
\uncommentfoo
\begin{foo}
  \item a
  \item b
\end{foo}
\end{document}
```

However, instaed of defining the two macros you can simply use

```
\iffalse
% This is hidden, i.e. comment out
% With \iftrue it is active
\begin{foo}
  \item a
  \item b
\end{foo}
\fi
```