A family of Microsoft word processing software products for creating web, email, and print documents.
Before that question can be answered completely, we need to know which kind of dropdown control your document uses: content control, legacy formfield, or ActiveX.
If it's a content control, the entries in a dropdown list control cannot be edited directly in the control. You can replace the dropdown list control with a combo box control. That allows the user to make a selection in the list and then edit it in the display box, or just type anything they want into the box regardless of the list entries. Alternatively, instead of changing to the combo box control, you can create a macro that asks which entry to edit and what to replace it with -- but that's no more convenient than the Properties dialog.
The legacy formfield dropdown list box is even less forgiving than the dropdown content control, because there is no combo box formfield as an alternative. If that's what you have now, I suggest changing from legacy controls to content controls and using the combo box. (The equivalent of restricting editing for formfields is to apply a Group content control around the controls and the surrounding text.)
The ActiveX controls offer a combo box but don't include a dropdown list box. The combo box defaults to allowing any entry in the display, but there is a property named "MatchRequired" that would prevent that if it is set to True instead of the default value of False.