// You said this dialogs parent is you app window, so you can use
// its hwnd as a parent where you will be searching for dialogs to close
// hwnds
HWND hApp = .....;

// iterate all childs of hApp windows
HWND hwnd = ::GetWindow( hApp, GW_CHILD | GW_HWNDFIRST );
while( hwnd ) {

   // Now check if this is the one you are looking for.
   // Instead of checking its class name you may check its window text,...
   // For checking class name / window name, you may use spy++
   CString sOut;
   GetClassName( hwnd , sOut.GetBuffer( 100 ), 100 );
   sOut.ReleaseBuffer();
   if ( sOut == .... ) {
      // it was found so, Post message to close it,
      PostMessage(hwnd, WM_CLOSE, 0, 0)
      // maybe SC_CLOSE
      // PostMessage(hwnd, WM_SYSCOMMAND, SC_CLOSE,0);
      // ... other way to close it? see here:
      // http://stackoverflow.com/a/12676807/471160
   }

   hwnd = ::GetWindow( hwnd, GW_HWNDNEXT );
}

do
 {
  hWnd = FindWindow(NULL /*class name?*/, /*windows name?*/);
  if(hWnd != NULL)
   PostMessage(hWnd, WM_CLOSE,NULL,NULL);
 }while(hWnd != NULL);