It it possible to scramble/distort a persons voice, so he becomes unrecognizable?
That’s not a yes/no switch. Somebody can transcribe the speech into printed words and someone else will still recognize favorite words that the speaker uses. Even with the pitch shifters, the meter and rhythm of the speaker goes through. Even if it doesn’t sound like Joe Smith, it sounds like me mum trying to imitate Joe Smith. I’m going with no.
I presume that you mean “and still intelligible”.
Try the Vocoder effect: http://manual.audacityteam.org/o/man/vocoder.html