I think that might be quite difficult to do programmatically as you would need some method of tracking anonymous visitors and counting how many forum posts they had been to. At the same time, you would need to add exceptions for to the counter for specific useragents like Googlebot.
I think the closest and possibly easiest method would be to modify the session handler. *nuke tracks anonymous visitors and puts their IP address in the nuke_sessions table so it might be realtively easy to add some additional code their to auto-block and IP after a given amount of time. Not very efficient but doable.
If you are only concerned with Google and you have a Google account, you can use their Webmaster Tools to give Google it's own user account on your site which it can use so it could crawl the forums even if you switch them to registered user access only.
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