How to alter Event Dropdown values from Register Participant or New Event Registration Page?
The event field on participant form uses event api to retrieve events. You can easily alter the list by implementing hook_civicrm_apiWrappers hook by adding additional parameter to exclude recurring event in $params of api probably 'parent_event_id' =&gt; ['IS NULL' =&gt; 1]. 

Or using buildform hook try adding parent_event_id condition in data-api-params of the field(element).

if ('CRM_Event_Form_Participant' == $formName) {
    $elementIndex = CRM_Utils_Array::value('event_id', $form-&gt;_elementIndex);
    if (empty($elementIndex)) {
      return;
    }
    $element = &amp; $form-&gt;_elements[$elementIndex];
    $dataApiParams = json_decode($element-&gt;_attributes['data-api-params'], TRUE);
    $dataApiParams['params'] = ['parent_event_id' =&gt; ['IS NULL' =&gt; 1]];
    $element-&gt;_attributes['data-api-params'] = json_encode($dataApiParams);
  }


Cheers

Pradeep