Normally a "clean win" refers more to the sportsmanship involved than the point spread: the winners played well, didn't engage in psychological tricks, didn't intentionally commit fouls, didn't argue with the referees, etc.
For beating the other opponent at zero, there are a variety of terms depending on the sport. Skunk is probably the most widely applicable across all sports, in the US. Shutout is also good in general, and predominant in baseball. Bagel (as a verb) is a term I've heard frequently in tennis when winning a set 6-0 ("He couldn't touch my serve, I bageled him in the second set").