Data return on Columbus when reset

I am selling my 2006 Skoda Superb and I want to wipe all the data from my Columbus Sat Nav system to avoid the new owner knowing personal I formation ( home address etc). Whenever I factory reset the system, all the data is immediately wiped but as soon as I use the system again, all the data has returned. How do I permanently delete all my data so it doesn't return? Thanks. Steve