Mark Beilfuss (just quit on top at a young age) and Chan Witt (died in a car accident) are the first two that come to mind. A few years ago there was the Engel kid from Minnesota, another great young player who just disappeared. A little further back we had Jon Kucharo from Iowa who was a world beater at 20-21 and out of competition by age 24.
The only time I saw Kucharo was in Vegas.
He got the 7-out from Parica and that might have made him quit.