Prohibit file deletion.

Hello again,
I want to prohibit all users except alfresco admins to delete files, even ones they created themselves. I've tried this:

Add somewhere near the top of the model/permissionDefinitions.xml file new permissionGroup definition and assign permissions by your needs:

then find:

and change it to:

but it gave me absolutelu no result. It seems that changes just not applying. What am I missing?