how can I fully disable resizing a window including the resize icon when the mouse hovers the border?

Qt has a windowFlag called Qt::MSWindowsFixedSizeDialogHint for that. Depending on what you exactly want, you want to combine this flag with Qt::Widget, Qt::Window or Qt::Dialog.

void MyDialog::MyDialog()
{
  setWindowFlags(Qt::Dialog | Qt::MSWindowsFixedSizeDialogHint);

  ...
}