I am not familiar with the Billion 7800 router, so I can't offer definitive advice. However, the easiest way to remove personal data from any router is to reset it to the original factory settings.
You will find reset instructions in the detailed user manual. If you can't find your manual, copies for the 7800N, 7800NL and 7800GZ routers are available at http://au.billion.com/product/usermanual.php.
Download the manual as a pdf file - and search the document for the word "reset". According to the manual for the 7800N, you will find a small Reset pinhole button, at the rear of the router (next to the power connector socket). To reset the router to its original factory settings, turn on the power. Disconnect the ADSL cable. Then, insert a paper clip or similar implement into the pinhole and depress the Reset button for more than 1 second.
If that doesn't work, you could use the router configuration menu to remove your personal details.
Use a web browser to navigate to the router's IP address at 192.168.1.254. By default, both the username and password are "admin". Click on OK to open the configuration menu.
Then, browse through the various options, until you find the Username and Password fields. Delete your data from these fields and save your changes.