to find the location of the field you want to unset, use the Devel Module:
Helper functions for Drupal developers and inquisitive admins. This
module can print a summary of all database queries for each page
request at the bottom of each page. The summary includes how many
times each query was executed on a page (shouldn't run same query
multiple times), and how long each query took (short is good - use
cache for complex queries). Also a dprint_r($array) function is
provided, which pretty prints arrays. Useful during development.
Similarly, a ddebug_backtrace() is offerred. much more. See this
helpful demo page.
then you could use hook_form_alter() to actually unset the value. to get the form information, something like:
hook_form_alter(&$form, &$form_state, $form_id)
dpm($form); // from devel module
if($form_id == FORM_ID_THAT_HOLDS_ELEMENT_TO_HIDE)
dpm will show the structure of any array, including the form - you can drill down into the data to find the exact location of the item you want to unset