Here are three options you can consider:
a) Make it a popup menu instead of a drop down list
b) Set a validation field option that only accepts a value if the value entered is a member of the specified value list
c) Use the OnObjectKeystroke trigger to perform this script which rejects all keyboard entry into the field:
Exit Script [False]
phIlMod…I knew it was right in front of me. As always thank you so much!