Fields can be disabled in customizing. In some cases though, this might not be sufficient. If for instance the requirement is that the field is in some cases disabled, for instance depending on another field or checkbox or radiobutton, you can redefine the GET_I method of this field.
If you wish to disable the field, insert the statement
rv_disabled = 'TRUE'.
If you wish to enable the field, insert the statement
rv_disabled = 'FALSE'.
Of course you can call other methods to determine the context and put some if-statements around the rv_disabled = ... statements to have the system determine the enabling/disabling of the fields correctly.