Today I was trying to work out some functionality on a form. After creating a new record of a case, I wanted users to assign a type to it, but once assigned, not allow them to change it. After a bit of digging on Google, I found the right method.
In the On Current event for the form, enter the following,
Me.AllowEdits = (Not Len(Me.Type & "") > 0)
Where Type is the field name you want them enter a value and then not change it. This seems to work just as I wanted it to – not bad for a single line of code. I love simple fixes.