Open Office, Word etc are word processors - they're designed for formatting text, and so pretty much by definition they're going to include formatting markup. If you want to write plain text for elsewhere, use a text editor, like Notepad (if you're on Windows) or Vim, Emacs or Gedit (if you're on an operating system, rather than a virus-support program).

I agree with AndrewHickey that using a text editor is best.

If you are really averse to writing HTML by hand though, there is Amaya, which is a web editor/browser that should generate very clean HTML if you don't explicitly set style properties. I haven't used it for years though and can't easily install it on Linux, but anybody on Mac or Windows should be able to install it fairly easily.