How to disable to update another user's document in kentico

As question above, how can I banned a user not to edit document created by another one. For example I have two users is user1 and user2, both users have a permission to created document. "user1" created a document on tree and then logout. I want when "user2" log on to cmsdesk, he can create and edit his document but he can't edit or read user1's documents.How can I do it?Sincerely