In Canada we have a pretty slick system to deal with overvoting like this. If you admit you messed up they can void the ballet in front of you and assign a new one.
If you don't admit or realize and cast an overmarked ballet it goes into the disqualified pile, your vote don't count.
Im from Canada and our PM ran on putting in a proportional ballet system, basically electoral college. He of course backed down on it, but the same people that hate the electoral college up here were huge supporters of proportional ballets.
It's mind boggling.