If you find a deer you don't have to call MNR anymore, you just fill in a form on the internet:
https://www.ontario.ca/page/keep-dead-wild-animal
A ball is property of an individual. A deer is property of the Crown until it is lawfully possessed by an individual. You can lawfully possess a deer by tagging it. If you can't get on the property, you can't tag it. You can also lawfully possess a deer by clicking on the link above and filling out a form.