You can use a script trigger on the field itself to test the condition on entering the field and exit the field if it's not met.
Be sure to check if you need an Exit Script (True) option in the script to let the cursor remain in the field, depending on which script trigger method you choose.
Tested - Worked good.
Then went with OnObjectSave script trigger to a script that checked the condition and reset the field to the proper value if the user changed it inappropriately.
Worked good as well.
Many hanks to you, Sir!