Making a Field Read Only
There are several techniques for preventing a user from entering or editing a field on a form.
Disable the field in its Properties dialog. Right click on the control, click Properties ..., and check the Disable check box on the Setup tab. You can do this interactively with the <OBJECT>.ENABLE() and <OBJECT>.DISABLE() methods.
Make the field inaccessible in Field Rules. Right click on the form's parent table on the Tables/Sets tab of the Control Panel, click Edit Field Rules..., select the field in the drop down list box at top, display the Data Entry tab, and enter ".T." for the Skip expression.