Apostrophe ending a word sticks to next word

Remove the activeacute option: if you use UTF-8 you don't need it.

\begin{filecontents*}{\jobname.bib}
@article{test1,
title   = "Designing conjoint choice experiments using managers' prior beliefs",
author  = "Zsolt Sándor and Michel Wedel",
year    = "2001",
journal = "Journal of Marketing Research",
volume  = "38",
number  = "4",
pages   = "430--444"
}

@article{test2,
title   = "Designing conjoint choice experiments using managers’ prior beliefs",
author  = "Zsolt Sándor and Michel Wedel",
year    = "2001",
journal = "Journal of Marketing Research",
volume  = "38",
number  = "4",
pages   = "430--444"
}
\end{filecontents*}

\documentclass[letterpaper, 12pt, oneside]{book}

\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc}

\usepackage[fixlanguage]{babelbib}
\selectbiblanguage{spanish}

\begin{document}

Hello world \cite{test1} and \cite{test2}

\bibliographystyle{babalpha-fl}
\bibliography{\jobname}

\end{document}

The filecontents* environment is used just to make the example self-contained and not clobber my files.

If you really need activeacute, type in the apostrophe as

Designing conjoint choice experiments using managers'{} prior beliefs

Various things like managers{'} prior did not work, but managers'\ prior did work.

\documentclass[letterpaper, 12pt, oneside]{book}

\usepackage[activeacute,spanish]{babel}
\usepackage[utf8]{inputenc}

\usepackage[fixlanguage]{babelbib}
\selectbiblanguage{spanish}
\usepackage{filecontents}
\begin{filecontents*}{bibliografia.bib}
@article{sandor01_conjoint,
title   = "Designing conjoint choice experiments using managers'\ prior beliefs",
author  = "Zsolt Sándor and Michel Wedel",
year    = "2001",
journal = "Journal of Marketing Research",
volume  = "38",
number  = "4",
pages   = "430--444"
}
\end{filecontents*}

\begin{document}

Hello world \cite{sandor01_conjoint}

\bibliographystyle{babalpha-fl}
\bibliography{bibliografia}

\end{document}