A couple of days ago I mused here about using server errors to kill spam/trackback bots. It seems I was not the first to think about this and David Angier has done work on porting Bad Behavior from Word Press to Drupal. I'd like to have some more configuration control but I'll likely figure it out shortly.
Update: The Drupal Bad Behavior module works wonderfully. Comments and trackbacks are on and are being filtered and real comments are untouched. There is no menu item for administering the settings. I added the following as the third menu item:
$items = array('path' => 'admin/settings', 'title' => t('bad behavior'), 'callback' => 'badbehavior_settings', 'access' => user_access('administer bad behavior'));
Update too: My earlier update seems to be unnecessary. It was likely a caching issue that prevented the menu from appearing. So far more than 800 spams have been prevented and there have been no false positives detected. A few folks have had the same problem I did which is that you have to install badbehavior.module in the bad-behavior directory in the modules directory or it won't find the include files it needs.