I have added the backpack to the default group. Is there a way to exclude individual players from the backpack? Some don't want to use it on our server.

No, since there is no way to revoke an Oxide permission that the player was given from a group that are in. What you could try is using a plugin that blocks all backpack commands from even being run, if the player has permission.

Thank you very much for the answer. Then I know what to do. I will create an extra group and do a bit of work. By the way, great plugin, especially since the enlargement.