Hide secondary category

is that you have v2.0.3 beta (if you have beta1 r1685 then please update to v2.03. beta2, it has 2 bug fixes that you need)

You should not hide "required" fields, if your field is already set a default value then e.g. for category field:

if (JRequest::getVar('Itemid', 0)== 45 ) {
$css = '
/* hide CATEGORY label */
#jform_catid-lbl {
visibility:hidden;
}

/* hide CATEGORY field */
#container_fcfield_name_catid {
visibility:hidden;
}
';
$document = JFactory::getDocument();
$document->addStyleDeclaration($css);
}

Add above to index.php of your joomla template, or to some other PHP file.

The above CSS code will be added when current Joomla menu item id is 45

-- SO make sure you replace 45 with the correct menu item id !!


-- BUT for item submit you do not need to hide it, in submit menu configuration you can make it obligatory to be a specific category, thus user cannot edit it