How do I prevent LaTeX from creating en-dash and em-dash?

-- and --- are TeX ligatures (like fl or fi). Disabling ligatures is one of the features of package microtype. You may deactivate all ligatures of all fonts:

\documentclass{article}
\usepackage{microtype}
\DisableLigatures{}
\begin{document}
---
\end{document}

or only the ligatures of the rm-family:

\documentclass{article}
\usepackage{microtype}
\DisableLigatures{family=rm*}
\begin{document}
---
\end{document}

or only the ligatures -- and --- of all fonts:

\documentclass{article}
\usepackage{microtype}
\DisableLigatures[-]{}
\begin{document}
---
\end{document}

or of one family, e.g. the tt-family:

\documentclass{article}
\usepackage{microtype}
\DisableLigatures[-]{family=tt*}
\begin{document}
\textrm{---} but \texttt{---}
\end{document}

See the manual of package microtype for more information about disabling ligatures.

You could use {-}{-} and {-}{-}{-} for en-dash and em-dash respectively, or define a command \dash that does this:

\documentclass{article}
​\begin{document}
\newcommand{\dash}{{-}}%
\renewcommand{\arraystretch}{1.2}
\begin{tabular}{lccc}
  & Default & \verb!{ }! & \verb!\dash! \\ \hline
  \verb!-! & - & {-} & \dash \\
  \verb!--! & -- & {-}{-} & \dash\dash \\
  \verb!---! & --- & {-}{-}{-} & \dash\dash\dash \\ \hline
\end{tabular}
\end{document}​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

I assume you'll be able to differentiate between math and text mode, since using {-} or \dash in math mode would mess up operator spacing.

\def\normalhyphen{-}
\catcode`-=\active
\protected\def-{\normalhyphen\ifmmode\else\kern0pt \fi}

You won't be able to specify negative dimensions by -, but you can say

\setlength{\mylen}{\normalhyphen 3pt}