Easy, just install an Alarm system or wire a relay for starter kill. So basically the car won't start unless the alarm is disabled by the key fob or activated by a hidden push button.
Heres your relay diagram if you scroll to the passive starter kill -- http://www.the12volt.com/12voltimages/starterkill.gif
And here's a cheaper basic alarm system with a starter kill built in -- http://www.sonicelectronix.com/item_18470_Viper-3105V.html