Obviously a large number of Cubans and Soviets from before the end of Communism weren't allowed to turn pro. From the US, we have a few options...one that comes to mind for me is Charles Mooney, Silver Medalist on the 1976 team at Bantamweight and never turned pro; only one from that team who didn't.
Can't remember names but so many Soviets during the Cold War-maybe the reason for the wave of champs now.
In 1973 there was a terrific US amatuer Heavyweight Champion, James Chapman, who went to medical school instead of boxing professional. Also, Igor Vytovsky, was one of the few fighters during Teo Stevensons prime to beat him, and by TKO. Vytovsky had trouble with cuts and the Soviets used other fighters for touraments, but Igor for dual meets.
Stevenson, could have been a great heavyweight in the pros..amazing fights we missed out on best right hand in the history of Boxing...