Hi,
The complete uninstall need to clean up the HikaShop tables (to drop them).
If you don't have access to your SQL interface (phpmyadmin for example), you can use the HikaShop override system in order to put the SQL queries to delete "manually" the HikaShop tables.
So, you can use a backend view, put the PHP code to clean HikaShop tables, call this special new view and then, uninstall HikaShop in the Joomla extension manager.
Then, when you will re-install HikaShop, the "new install" process will be perform and you will have a all new install.
But like lousyfool said (and Nicolas in the other thread), using a mysql administration interface is better.
Regards,