Is it legal to discipline an employee for tardiness by suspending her without pay?

Q. I work in HR at a customer call-in center. To make sure we have enough coverage to handle calls, we have a strict tardiness policy. Recently, one of our customer service agents was late for work several days in a row. She is an otherwise outstanding performer and we don’t want to fire her. In the alternative, we would like to suspend her for one week without pay. Is that legal? ...