#A Combobox is a tkinter.ttk widget equivalent to OptionMenu of tkinter but better than it. But there is a text entry in it so here is the code to disable it :
from tkinter import *
from tkinter.ttk import *
root=Tk()
selected=StringVar()
cb=ttk.Combobox(root,textvariable=selected,values=["Car","Bike","Scooter"])
cb.pack()
root.mainloop()
#NOTE : It is necessary to write from tkinter import * and then in next line from tkinter.ttk import * to get both tkinter and tkinter.ttk widgets.