Lockit is an Accessory for ExpressionEngine 2.x and will allow you to lock down any fields that you don't want to be editable once populated.
To install, simply put the lockit directory inside your system/third_party/ directory then activate within your ExpressionEngine control panel.
Instructions for use
To lock a field, just add "Lockit" to the field's label (not its short name).
If you have a field labelled "Favourite Cheese" then just change its name to "Favourite Cheese Lockit"