To stop google indexing forum posts the only reliable way is to ensure the forums are not visible to the "Anonymous" member group within InstantForum. Google can only index and see posts that are within forums visible to Anonymous users.
You can control the member groups that can access a forum from the Edit Forum page within the Admin CP.
I learnt recently Google actually ignores your Robots.txt most of the time.
The robots.txt is commonly used for folder level disallow rules. If you wanted to restrict Google indexing a specific folder one option may be to add a <authorization> element to your web.config so only authenticated users can access specific folders.
For example say you had a folder called "Members" and you only wanted to allow users who are authenticated via forms authentication access to this folder you could add the following into your web.config file...
<allow roles="Administrators,Forum+Members,Moderators,Awaiting+Activation" />
<deny users="*" />
Any requests to /members will return a 200 for authenticated users or the server will return a 302 redirect if you don't belong to the roles. For example your the Google Bot.
I hope this helps provide another option.
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