As the others mentioned earlier, you can't restrict it so it can't be taken. I would also suggest a CHANGED_LINK event. something like:
changed (integer change){ if (change & CHANGED_LINK) { agent=llAvatarOnSitTarget(); if (agent==NULL_KEY) { llDie(); llSetTimerEvent(1); } }}timer(){ llDie();}
I might have messed up somewhere since I just made this up without testing.) Which simply deletes the vehicle when the driver stands up. Oh and of course make sure to set the permissions for your vehicle to "no copy".