How to prevent LaTeX from hyphenating the entire document?

This is discussed in detail in the TeX FAQ. Summarising the information given there:

1. You can set \hyphenpenalty and \exhyphenpenalty to 10000, which will stop hyphenation, but as TeX will still try to hyphenate this is not hugely efficient.

2. As Joel says, you can use \usepackage[none]{hyphenat} to select a 'language' with no hyphenation at all. This works fine for a single language document, but not if you want to use babel or polyglossia for other language-specific effects.

3. Setting \righthyphenmin and \lefthyphenmin to very large values will prevent hyphenation as it tells TeX that it must have more characters in the word than are going to be available. The suggested value in the FAQ is 62.

4. You can set \hyphenchar\font=-1, which will prevent hyphenation for the current font: this is probably not the best way for an entire document but is how it is done for the tt font shape in LaTeX.

Now, of those (2) is probably the best choice. However, what you did not say is why you want no hyphenation. TeX hyphenates when it cannot find a good line break without it, so you get few hyphens in most cases. The risk with no hyphenation at all is that the output looks bad.

I use this and it works great for me in almost all documents:

\tolerance=1
\emergencystretch=\maxdimen
\hyphenpenalty=10000
\hbadness=10000

\begin{document}... yadda yadda\end{document}

A quick google found

\usepackage[none]{hyphenat}

and more useful info here.