It's not possible to prevent the renumbering without some alterations of the import script. The mapping will never go away since it's stored in your local database in the ubbt_IMPORT_MAP table.
The importer was written to allow for multiple imports. The site_id is basically an import id. Since you could actually import several forums into your version 7 installation, and each would have it's own site_id so it knows which post to lookup.
