you can flash the RUU thats in the dev forums, it should wipe the phone.
i haven't tried to wipe this phone yet, but on my gnex, iirc, there was an option when you asked it to wipe to remove all personal data as well. i'll have to check for it on my 1x after i finish flashing the new RUU to it.
i looked again, it's in settings / storage. in there it has options to wipe saved data from the apps partition as well as the sdcard partition, and also the factory wipe.
Lumia 920 - White WP8
HTC One 32GB