If you like to fix all the fields on the checkout form. Let’s suppose customer placing the additional order which may confuse you if he has filled different details then you can fix/disable all the fields as shown below:
Note: Make sure that you add the readonly attribute to disable the fields if you use disabled attribute in it then it may start throwing error that the fields are required as Disabled <input> elements in a form will not be submitted!
If you like to fix/disabled some fields only so you can do that as shown here.