Preheat the barbecue to medium-high. Brush the meat portion of the roast with the olive oil, then sprinkle and rub on the Cajun spice. Set the roast, bone side down, on one side of the barbecue. Close the lid and turn the heat off on the side of the barbecue the beef is sitting on; leave the other side set to medium-high. Cook the beef until the desired doneness is achieved. 60 to 70 minutes should cook it rare to medium rare; 70 to 80 minutes to medium.