(In reply to Kunal Mehta (Legoktm) from comment #3)
Makes sense. In that case, you can just add a hook for
'TitleQuickPermissions' in wmf-config/CommonSettings.php to deny any user
the import right in the item and property namespaces.
Sadly this is by far not this easy as core is only checking for the permissions in general, but not whether the user can import to a specific title (or namespace). I did some initial hacking in Wikibase but I'm not sure how to adopt core (that's super evil legacy code again...).
Mhm, is there any reason why this is such a high priority bug? I mean, since the import right is restricted to administrators anyway, and these are usually acting responsibly, a simple notice on Special:Import asking them not to import from ns0 should do it as well until a solution like Legoktm proposed becomes real instead of removing functionality (import of non-items) from the wikis.