There are various ways to do this, but the best is to use an advanced CSS theme:
http://www.question2answer.org/advanced.php#theme-advanced
So, in the theme.php file in your theme directory, try the following:
<?php
class qa_html_theme extends qa_html_theme_base
{
function main()
{
global $qa_login_level;
if (($this->template=='users') && (@$qa_login_level<QA_USER_LEVEL_ADMIN)) {
// do nothing...
} else
qa_html_theme_base::main();
}
function nav_list($navigation, $navtype)
{
global $qa_login_level;
if (($navtype=='main') && (@$qa_login_level<QA_USER_LEVEL_ADMIN))
unset($navigation['user']);
qa_html_theme_base::nav_list($navigation, $navtype);
}
}
?>
If the viewer is not logged in as admin, the first function above makes the users page blank, and the second function removes the user entry from the navigation menu.
By using a theme, your solution should still work if you upgrade Q2A in future.
(BTW I haven't tested the code above so apologies for any bugs...)