act_wiz.c function do_khistory
find

   one_argument( argument, arg );

change that to this

   argument = one_argument( argument, arg );

then find

   for( track = 0; track < MAX_KILLTRACK && vch->pcdata->killed[track].vnum; track++ )

above that add

   if( get_trust( ch ) >= LEVEL_IMMORTAL && !str_cmp( argument, "clear" ) )
   {
      for( track = 0; track < MAX_KILLTRACK && vch->pcdata->killed[track].vnum; track++ )
      {
         vch->pcdata->killed[track].vnum = 0;
         vch->pcdata->killed[track].count = 0;
      }
      send_to_char( "   &RHas been cleared.&D\r\n", ch );
      return;
   }

You can of course change LEVEL_IMMORTAL to what ever you want it to be.