[Jochemd]

pawn Code:

if(newstate == PLAYER_STATE_DRIVER) // In OnPlayerStateChange, when he just entered the vehicle.
{
    new skin = GetPlayerSkin(playerid);
    switch(GetVehicleModel(GetPlayerVehicleID(playerid))
    {
        case 427, 596, 598, 597, 599: // Here the vehicle models
        {
                if(skin != 280) // Add more skins yourself
                {
                    SendClientMessage(playerid, COLOR_RED, "You have to be a cop to enter this car!"); // Send a message he isn't a cop
                    RemovePlayerFromVehicle(playerid); // Remove him from the vehicle
                }
        }
    }
}