Using bellow php code you can make ACF custom field as Read only.
Place bellow PHP code in your theme functions.php file
/**
* Read only input fields in Admin ACF Group
* custom_field_name
*/
function disable_custom_field_name_load_field( $field ) {
$field['readonly'] = 1;
return $field;
}
add_filter('acf/load_field/name=custom_field_name', 'disable_custom_field_name_load_field');
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