You could remove the 'switch_themes' capability from the 'administrator' role, which should prevent regular admins (as opposed to the network superadmin) from being able to switch their theme. Try this:
add_action('admin_init', 'custom_remove_switch_themes_role_from_admins');
function custom_remove_switch_themes_role_from_admins(){
global $wp_roles;
$wp_roles->remove_cap('administrator', 'switch_themes');
}
EDIT:
From a bit more extended testing with this above function, I noticed some issues when I removed a cap and then did not explicitly add it back again when I was done testing (it seemed to actually remove it from the database rather than just for the current page). This is alright, but just be careful! If you are less comfortable with that, try the following, which intercepts capabilities for all roles, not just admins (note that network admins are excluded from this, since they have all capabilities):
add_filter('user_has_cap', 'custom_remove_switch_themes_capability', 10, 3);
function custom_remove_switch_themes_capability($all_caps, $caps, $args){
if ('switch_themes' == $args[0]) // You can chain additional caps (using or) here if desired
return array();
else
return $all_caps;
}