def defocus(event):
    event.widget.master.focus_set()
root = Tk()
comboBox = Combobox(root, state='readonly', values=('a', 'b', 'c'))
comboBox.grid()
comboBox.set('a')
comboBox.bind('<FocusIn>', defocus)
mainloop()
How to control the tkinter combobox selection highlighting