Disallow access from psql, or allow access only from specific client app

Hi everybody:
I searched the web trying to find an answer to this, but found none. I have a postgresql server and a database, and I granted access to some users.
However I want them to access the data only through some specific client application. I do not want them to have access through psql or some other kind of client app. But, as I created them a user/pass they could use them.
What would you do?
Thanks in advance.