Yes, but my question, I guess... Is how would they know that the person was in Canada for longer than they were supposed to be?
I'm not asking because I'm considering this... I'm really just curious.
Well to be honest with you they don't, Canada doesn't really have any exit controls. Canada will not be able to find out when that person left Canada and entered the U.S, as the U.S doesn't stamp passports of returning citizens. Even if they scan it as Baloo said, I doubt Canada has access to that information. I don't think the U.S would give Canadians information on when every specific American citizen enters the U.S from Canada, unless it was specifically requested as part of some high profile case. I don't even think the U.S keeps detailed records of when it's citizens enter from Canada, crossing the border with my sister (U.S citizen) when we entered the U.S, the border guard simply looked at her passport and gave it back to her, no stamp or scan.
So to answer you question, I guess nothing really stops them except for the fact that most Canadians and Americans are good honest people.