"Do not eat that apple!"
Translation:O elmayı yeme!
It would have to be elması for one. Secondly, that would mean "his/her/its apple" and "o elması yeme" would be ungrammatical. "Don't eat his/her/its apple" would have to be "onun elmasıyı yeme", with the possessive form of o and the accusative of elması, since it's now a specific apple.