T-Mobil charge me penalty fee for early plan termination. But in fact my plan had already expired before I canceled it (confirmed by 2 T-mobil stores). I ask for proof of contract which has my signature showing the starting date of the plan but they don't have one. How could I refuse to pay for this wrong charge?

You just simply don't pay. They will probably report this to the credit bureau and they might also take you to Small Claims Court. You must have cancelled checks or bills showing when the phone was activated. Use those as proof of the starting date of the contract.

Avoid Court by sending copies of those checks directly to T-Mobile.