Some ideas for controlling search engines:
Deleted blogs' robots.txt
All deleted blogs should have their robots.txt set to Disallow: /
Wp-login index
Stop wp-login.php from being indexed by adding <meta name="robots" content="noindex" />
Disable archiving
An option to disable archiving by search engines by adding <meta name="robots" content="noarchive" /> and Disallow the ia_archiver robot.
Disable pings
An option to disable pings to ping servers (such as weblogs.com.)
Disable feeds
An option to disable feeds should help to keep out some bots, (although it's quite a sacrifice.)