Dear Kalpana,
Punching/swiping another employee's card or signing on behalf of another employee is a far more serious offense and shouldn't stop at just giving a warning letter. If you only give a warning letter, it will send the wrong message to everyone that management takes a lenient view of this type of misconduct. I recommend conducting a domestic inquiry, and if the employee is found guilty, then they may be suspended for 1-2 days or have their wages forfeited for 1-2 days.
You may want to reconsider your decision.
Ok...
Dinesh V Divekar