Once a user groups Nominal Code access has been set some users may still have access to either all Nominal Codes or other codes that they have not been given access too.
This is due to the user having a higher nominal code access then the user group, you can check this by going to Accounts > Purchase Ledger > Ledger Parameters and click coding permissions.
Search for a user and double click on their name.
Here you see what their current access level is. As shown in the screenshot the user has access to all Nominal Codes meaning any access set on the group it will override.
Make sure that the All box is anticked and no other range of codes have been set.