how to effectively hide a field (with title) on a form
You can set the #access property for the form field to FALSE. The form field will not be shown to the users, but its value will be still be available in $form_state['values'] for the submission, and validation handlers.

Using #access is always the suggested way to avoid users can change the value of a form field, and avoid any third-party module depending on that form field starts to raise warnings, or fails to work as expected.