[JesterlJoker]

There are two simple ways on that

OnPlayerEnterVehicle

and OnPlayerStateChange

I only can give OnPlayerEnterVehicle example
i.e

now since you have already have that scar[MAX_SCAR] if that's how that variable is made, I'll go with my idea of how it's made

first you make this piece of function

PHP код:

stock IsScar(carid) //check if the car is a scar vehicle
{
    for(new v = 0; v < sizeof(Scar); v++) {
        if(carid == Scar[v]) return 1;
    }
    return 0;
}

public OnPlayerEnterVehicle(playerid, vehicleid, ispassenger)
{
    if(!ispassenger) //check if the player is not a passenger
    {
        SetPlayerArmedWeapon(playerid, 0);

           if(IsScar(vehicleid)) 
           {
               if(Player[playerid][ScarOwner] != vehicleid) //this would just be example for removing the player 
               {
                RemovePlayerFromVehicle(playerid);
                new Float:slx, Float:sly, Float:slz;
                GetPlayerPos(playerid, slx, sly, slz);
                SetPlayerPos(playerid, slx, sly, slz);
                NOPCheck(playerid);
                SendClientMessage(playerid, COLOR_GRAD2, "You can't right this car");
            } //this way the player would be removed if he does not owned that scar vehicle
        }// you can make it if the player is not the leader of the group the scar is supposed to be used they get ejected
    }
} 


this can be improved with much more security if you want so that not even a hacker can move this cars