The dunk was banned from 1967 to 1976 in College Basketball, according to Wikipedia.org. I'm not sure it was ever banned in pro Basketball, except that the players used to just lay it up rather than dunking in fear of retribution (in the form of hard fouls), if I remember correctly.
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From 1967-1977 were the only years the dunk was not allowed in the NCAA.
It has always been legal in professional basketball