We can rate the mod, so why can't the mod rate cup players?
If a player has a bad rating then he can't join a cup. Unless he decides to take it seriously then a second chance can be given to him.
Kinda like the warning nino suggested
For example : a player doesn't play any games in the cup, he gets a 0 from the mod at the end of the cup (If no health or study excuses of course, mod could replace if such situation)
A player who played all his games get a 5.
If you are under 3, a mod will decide if you can join the cup or not.
Under 2 you are fobidden to play cups unless you promise to play all your games and hold this promise. If not banned from cups.
There are too many people who join cup and don't play it, maybe they just like to click on join button
With such system we could get rid of all usual inactive players.