To keep users from typing data into a field formatted with a drop down list, use one of the following techniques:
a) switch to a pop up menu instead
b) In Field Options for the field formatted with this value list, set the validation option that limits values to only those that are a member of the value list.
c) Set up the OnObjectKeystroke script trigger on this field to perform this script: Exit Script [False]
I believe that c) above will work on date fields though I haven't tried it.