A term for a "clean" win

Solution 1:

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").

Solution 2:

A good word for what you're looking for is shutout. It is defined by Merriam-Webster as:

A game or contest in which one side fails to score.

It can be used in other contexts as well (such as bridge, apparently, where it holds a different meaning), but in reference to sports it is pretty unambiguous.

Solution 3:

Other terms you might consider: walkover, whitewash.

Walkover does have a more specific meaning -- it is used in some sports to mean that the contest has been awarded to one competitor because the other competitor has failed to show or has otherwise forfeited the contest. But by extension, it has also just come to mean "an easy victory".

Solution 4:

To-nil and clean sheet are general terms indicating that one team has not scored. However, when large gaps occur there are lots of idioms; for example, a few include thrashing, pasting, drubbing, and routing.