I've used IPB 2.x and I've upgraded to 3.x last year after this, I got a lot of troubles.
I was very careful to block in robots.txt any thin content page.
The problem is that you must understand that blocking via robots.txt does not means that the pages will not be indexed!
So.. quick resume what to do:
1. Go to Google WMT (Index - Remove URL) and drop those directories from your forum:
1.1 Go to ACP - Members - Member Groups - Manage Member Groups - Edit Guest - Global Tab
Can view member profiles and member list? - NO
Can use search system? - NO
If you do not want to deactivate this function, then edit your profile template and put rel="nofollow" to the Website URL
Default it has: rel="external me".
3. Hide forum signatures from guests (settings, Googlebot is Guest)
ACP System Settings > Members > User Profiles
Allow guests to view signatures - NO
If you still use IPB 2.x, then you must use Google WMT - Crawl - URL parameters in order to block a lot of unusefull parameters.
Take each one by hand, study the examples.