How do I edit activity forms?
Graham, there are loads of tutorials on how to use jQuery. The CiviCRM approach would be:


Identify the ID of the field to hide using FireBug (or the inspect element function of your browser)
Add a template with the jQuery required to hide the identified id's which will be something like this:


{literal}
  
    cj(document).ready(function() {
      cj('#firstTestID').hide();
      cj('#secondTestID').hide();
    });
  
{/literal}


Add a hook in an extension to add the template with your hides to the template with something like this statement:
        CRM_Core_Region::instance('page-body')->add(array('template' => 'GroupProtect.tpl')); OR add the jQuerys to a .extra template (in the documentation https://wiki.civicrm.org/confluence/display/CRMDOC/Page+Templates)


Does this help?