Avoid line break between em-dash and the next word

Since you are writing in Spanish, you surely are using the spanish option for babel. If this is the case, then you can use the shorthand "+-- to prevent a possible line break. A little example:

\documentclass{article}
\usepackage[spanish]{babel}

\begin{document}

%wrong output:
\noindent Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam a leo quis 
 ---accumsan vulputate---. Praesent pretium dapibus est interdum 
hendrerit.

%right output:
\noindent Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam a leo quis 
 "+--accumsan vulputate"+--. Praesent pretium dapibus est interdum 
hendrerit.

\end{document}

​

Another option is to use the \nobreakdash command implemented by the amsmath package; this command suppresses any line break after the dash (or hyphen). Of course, if you are going to use this a lot, you can define a new command to simplify the writing. In the following example I defined two commands using \nobreakdash: the first variant will prevent hyphenation of the word following the em-dash; the second variant will allow normal hyphenation in the following word:

\documentclass{article}
\usepackage{amsmath}

\newcommand\RayaN{\nobreakdash---}
\newcommand\Rayan{\nobreakdash---\hspace{0pt}}

\begin{document}

\noindent Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam a leo quis 
 ---accumsan vulputate---. Praesent pretium dapibus est interdum 
hendrerit.

\noindent Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam a leo quis 
 \RayaN accumsan vulputate\RayaN. Praesent pretium dapibus est interdum 
hendrerit.

\noindent Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam a leo quis 
 \Rayan accumsan vulputate\Rayan. Praesent pretium dapibus est interdum 
hendrerit.

\end{document}​

You can set \exhyphenpenalty=10000. After this, the hyphen-like places (generated by - or -- or --- are unbreakable.

Advantage: The setting can be done only once at beginning of the document. You needn't to change the markup of the document.

Disadvantage: the word immediately followed after --- is not hyphenated. If this brings the problem, you can insert explicitly hyphen points in this word by \-.