One way is just use a file manager to delete your files. Another is in Philz, you can do an advanced wipe if I remember correctly and choose to wipe the internal.
Factory reset never wipes internal.
Yet another way is to Odin your current or never version of official TouchWiz to your phone, that normally does wipe internal.
Sent from my Sprint Samsung Galaxy SIII on TouchWiz 4.4.2 with Wicked X v6.5, Ktoonsez' latest kernel and the latest Philz Touch Recovery.