I had the same problem. A very simple solution is to add these in the robots.txt:
Disallow: /user*
Disallow: /register*
Disallow: /login*
Disallow: /ask*
Disallow: /forgot*
Disallow: /chat*
Disallow: /admin*
Disallow: /activity*
Disallow: /questions*
EDIT: I just changed the above rules to make it more efficient.
