First you must decide what member group you want to allow or disallow.

Then after logging on to the admin cp go to Users and groups > Manage users group > (group you want to edit)

After you go to the page that allows you to edit each group you must then go to the option that says "Allow user to remove 'Edited by' legend?" switch it to yes or no, whichever you want for that member group. Then when you finish that step you must click on complete edit.

If you like you can go over and do the same thing for the other member groups you want to edit.