Making a window not resizable in Windows
Here are a couple of ideas:
You can try removing the WS_THICKFRAME style:
frame.ModifyStyle(win32con.WS_THICKFRAME, 0)
Another alternative is to handle the WM_GETMINMAXINFO message that returns the minimum and maximum width and height for a window. In your case, you would return the current width and height. You can get this information with GetWindowRect.