You can set user rights to avoid any modification on Item table. On user role, for the Item table, you need to set permissions:
-Read: YES
-Insert: YES
-Modify: -
-Delete: -
If you want to user can make modifications on Item tables on other fields, you can make the same permissionos for Item Ledger Entry, because when you try to modify Item No., system will modify this field in all ILE entries. If you have not posted entries, system will allow rename.
If this is not enough, you have to make customization.