This is what Sunny Day Online does to control Spam in Drupal 5.0 with anonymous commenting enabled. I decided to publish this here to share with the Drupal community and also with our staff.
1) Use FormFilter module to remove homepage field and preview button.
A) Download module from Drupal.org and install it to sites/all/modules (http://drupal.org/project/formfilter).
B) Enable FormFilter module in admin/build/modules.
