Hi Soeren,
BADI name is CRM_ORDER_FIELDCHECK.
Implement this BADI and code in method: FIELDCHECK
There will be changing parameter: CT_INPUT_FIELD_NAMES
READ TABLE ct_input_field_names INTO ls_input_field_names WITH KEY fieldname = <field_name>.
ls_input_field_names-changeable = ' '.
MODIFY ct_input_field_names FROM ls_input_field_names.
Regards,
Karthik.