Hiding marginal notes

There a number of methods you can use. One way is to use yet another package such as the todoor you can simply define a command as shown below, which will perhaps also be more semantically correct for notes.

\documentclass{article}
\usepackage{lipsum,xcolor}

\def\noteson{%
\gdef\note##1{\mbox{}\marginpar[$\leftarrow$ ##1]{%
    \color{blue}$\leftarrow$ ##1}}}
\gdef\notesoff{\gdef\note##1{}}
\noteson
% Comment line above and uncomment to hide notes
%\notesoff
\begin{document}
\note{Adams disagrees on this issue.} This is some lipsum text
\lipsum[1-7]
\note{Check the reference out.} This is some lipsum text
\end{document}

You can switch them on and off using \notesoff or \noteson.

If you want to use the todo package check the post How to add todo notes?

Just redefine the command to do nothing:

\renewcommand{\marginpar}[2][]{}

You could use the comment package (http://www.ctan.org/pkg/comment). A little example (change \includecomment{note} to \excludecomment{note} to hide the margin note):

\documentclass{book}
\usepackage{lipsum}
\usepackage{comment}

\includecomment{note}

\begin{document}

text text text text
\begin{note}
\marginpar{a margin note}
\end{note}
text text text

\end{document}