Every bug has been assigned to a specific user who belongs to specific
group.

These group has very limited access ( and not a member of any other
group), but the users of this group still can edit bug fields.

I want to customize some text fields/areas for those users, so that
they can not edit that specific field.

I also want to customize some text fields/areas for for Bug Creator
(who files the new bug) so that he/she can not edit his/her posted
bug.
(I also want to hide some text fields and want to add new text
fields )

I had a look at this document and make the following changing
(comments) in Bug.pl file but nothing happens.

*************************************************************************** *********
    # Allow anyone to change comments.
    #if ($field =~ /^longdesc/) {
    #    return 1;
    #}

    # At this point, the user is either the reporter or an
    # unprivileged user. We first check for fields the reporter
    # is not allowed to change.

    # The reporter may not:
    # - reassign bugs, unless the bugs are assigned to him;
    #   in that case we will have already returned 1 above
    #   when checking for the assignee of the bug.
#    if ($field eq 'assigned_to') {
#        $$PrivilegesRequired = 2;
#        return 0;
    }
    # - change the QA contact
 #   if ($field eq 'qa_contact') {
 #       $$PrivilegesRequired = 2;
 #       return 0;
    }
    # - change the target milestone
 #   if ($field eq 'target_milestone') {
 #       $$PrivilegesRequired = 2;
 #       return 0;
    }
    # - change the priority (unless he could have set it originally)
   # if ($field eq 'priority'
    #    && !Bugzilla->params->{'letsubmitterchoosepriority'})
    #{
    #    $$PrivilegesRequired = 2;
    #    return 0;
    #}
    # - unconfirm bugs (confirming them is handled above)
  #  if ($field eq 'everconfirmed') {
   #     $$PrivilegesRequired = 2;
   #     return 0;
   # }
    # - change the status from one open state to another
#    if ($field eq 'bug_status'
 #       && is_open_state($oldvalue) && is_open_state($newvalue))
  #  {
  #     $$PrivilegesRequired = 2;
   #    return 0;
    #}
*************************************************************************** **************

Is it possible what i want to do ?

Can we customize these fields via making changes in code ? or by Admin
side ? or by installing some plugins ? or any other way ?

Thanks in advance.