If you quit out before the credits you can avoid getting the prelude level.

If you do get the prelude level you have to find the save folder.

You can find the file here: C:\Users%username%\AppData\Local\VirtualStore\Program Files (x86)\Surreal\The Suffering\Saves

• Delete the save folder and "Unspeakable.cfg" This will reset your keybindings so make sure to set them again to your preference.