I have looked but have had no luck with this at all. I cannot see at all where Buddypress keeps its list of recognised blogs.
To add to this, I have also discovered that both WPMU and Buddypress seem to suffer from this disease … so it might be more WPMU related. I found older blogs and posts which should have been deleted still existing in my database.
That is just a guess though.
Is there a good way/plugin to clean out all inactive cruft from WPMU/Buddypress? Does BP even have a working uninstaller? I might have to rebuild from scratch … which I was trying to avoid.