If you could go back in time and stop any one person or group's musical career, who or what would it be?
This isn't a "go back in time and assassinate" someone question. It's just their musical career never takes off and thus any music they ever would have made is no longer in this timeline.
Outside the box: Robert Elliot, a charter pilot who flew and killed Jim Croce by crashing into a tree on takeoff. Pilot error was found to be the cause as the overweight 57 year old pilot was unable to drive to the airport so ran for 3 miles, from his motel to the hangar, to be on time for the flight. The run exacerbated his coronary artery disease, causing him to tunnel vision and not see the tree.
Similar story, when I was 12 I was late for the school bus and ran about 2 blocks with a full backpack and a snare drum. Ended up puking in the bus aisle about halfway through the hour long ride. Basically the same thing.