There are a few text editors that work like typewriters, e.g. Typewriter: Minimal Text Editor: "All you can do is type in one direction. You can’t delete, you can’t copy, you can’t paste. You can save and print. And you can switch between black text on white and green on black; full screen and window." It's Java but Mac-only, so I can't test it, but I'd be amazed if there weren't something similar for Windows or Unix. I gained huge writing skills churning out multi-page first-draft letters to friends on an acoustic typewriter; I need to get back into this mode of writing.

Edit: On a Unix box, cat>tmp.txt does pretty well :-)