Even thought it is not something that may be as successful as you might think, (users make mistakes and need to make changes to data, even the data they just entered) you can prevent changes to the data in any control on you form by setting its "Locked" property to true.
Use the After Update event of the control and place code like the following:
If Not IsNull(me.NameOfControl) then
Me.NameOfControl.locked = true
end if
this will lock the specified control if it had any data.
Now, next question, how do they correct a mistake?
Just my thoughts!