According to section 2.4 of the harvard manual, the double-starred (!) macro version \citeasnoun** overrides the default citation style and always uses the abbreviated form. It seems, however, that this feature is available for the six styles "currently available within the harvard family" (manual, section 3.1), but not for the econometrica style which, according to the notes in econometrica.bst, was created with the help of makebst. The following example shows the forcing of the abbreviated citation form using the agsm style.
\documentclass{article}
\usepackage{harvard}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@misc{A01,
author = {Author, A. and Buthor, B. and Cuthor, C.},
year = {2001},
title = {Alpha},
}
\end{filecontents}
\begin{document}
\citeasnoun**{A01}
\bibliographystyle{agsm}
\bibliography{\jobname}
\end{document}
