Isolate/protect user data from website


We need to somehow isolate or protect a website's users (signed up users) from the website itself. Meaning, the info the site will be recollecting, will be quite sensitive, and cannot be available to admins or developers of the site.
We were thinking of creating special admin roles (developers) who will not have access to the site's users, however, we don´t know how to prevent them from installing plugins that access the DB, where this data can be retrieved.

What would you suggest?