The metal sleeves are to shield the spark plug wire ends they are supposed to come off with the spark plug wires when you remove the spark plug wires. You need to make a strong hook to get to the bottom of the shield and pull it off or use a long needle nose pliers to remove them. The trick is when reattaching the spark plug wires after you replace the plugs is to use lots of dielectric grease on the porcelain of the spark plug body so the plug wire doesn't bond to the plug. I normally see this problem with cheaper spark plug wire sets. Once you wiggle and pull on them you stand a high risk of damaging the spark plug wires!