Okay let the mall charge if they feel that some non-customers are using the parking facility which marrs its genuine customers right. Try following and if that don't work apply fees from zero hour to xxx number of hours.
Parking should be free for 2 hours during week days and 1 hour during weekends. Anybody who has come for shopping should finish their shopping within 2 hours and someone wants to shop for more than 2 hours pay for extra hours. Genuine shppers won't take more than 2 hours in any circumstances.
Like in the airport at the time of entry the camera should issue ticket and count time at the time of exit.