Are you wanting to keep the sprocket? If not get a dremel and a thin cutting disc. Be very careful. Put a thin cut at 12 o'clock and another at 6 o'clock on the sprocket. But just so your through the sprocket. Then grab a screw driver and place it in one of the slots and twist. should break off in 2 bits, just be very careful. Done it once, but it will be okay if you don't rush it.