[MickDoneDee]

Inside the modcp folder open the index.php file. Look for this code and delete the section in yellow:

// *************************************************
$canmass = false;
if (can_moderate(0, 'canmassmove'))
{
$canmass = true;
construct_nav_option($vbphrase['move'], 'thread.php?do=move', '<br />');
}
if (can_moderate(0, 'canmassprune'))
{
$canmass = true;
construct_nav_option($vbphrase['prune'], 'thread.php?do=prune');
}
if ($canmass)
{
construct_nav_group($vbphrase['thread']);
construct_nav_spacer();
}

print_nav_panel();

echo "</div>\n";
// *************************************************

This removes the Prune link in the Threads section of the Navbar. Without the link they can't access thread.php to do the prune. This is not full-proof since a clever mod can call-up the prune function by typing /modcp/thread.php?do=prune in the address field of their browser.

As an additional security measure, either remove thread.php from the modcp folder, as Colin suggested or simply rename the thread.php file. I renamed it to threed.php so "The page cannot be found" appears when Prune or Move is clicked in the navbar. That is the downside of this suggestion - it also prevents Move threads, as well.

I suppose you could password protect that page using .htaccess or a hack like Password protect non vb pages.

Warning: .htaccess appears to only password protect directories (not individual pages) and is hidden when uploaded to your FTP server so can't be deleted. I had to delete the contents of the .htaccess and .htpasswd files and re-upload them to enable mod access again.