You could add Validation to the field to only be one of the values in the value list you have in your drop-down list.
There are a number of ways to do what you are wanting. Mostly it will depend on how you have things designed now and what kind of user experience you want for your users. Some Options:
- Change the field to a pop up menu instead of a drop down list.
- Use Validation to make sure the contents of the field match the items in the value list (still allows typing, but makes sure only the values you want are typed)
- Turn on type ahead (allows typing, but auto fills so less of a chance to get the wrong value)
- Capture keyboard input through an OnObjectKeystroke script trigger (requires a script to process the keystrok and act accordingly)
- Turn off access to the field in Browse mode and use a button to display the value list items (in a pop-over, or a picker, or a different field)
Beware, users can still paste into a field that has a pop-up menu.