ip_block

This module detects web bots visiting your site and automatically blocks them from commenting or logging in.
It can drastically reduce the amount of comment spam.

How it works

When a spam bot first visits your site it crawls looking for 'comment' links. At this stage the IP Block module detects it and records the IP address. Any attempt to log in or comment from this IP is blocked.

More details

The module puts a link on every node that points to a page created by the module. This link is hidden from normal users. Any visit to this page adds the IP address to a table. An attempt to comment or log in from an address in this table is blocked, and a message shown that directs the user to a simple captcha. If the capcha is completed correctly the IP address is removed from the table.
If anonymous comments are allowed, then when an IP is blocked, any anonymous comments from the same IP address are deleted. The module also prvoides a drupal 'block' that shows the blocked status of the current IP address.
The capcha is configurable and you can manually add and delete IP addresses as well as see a list of blocked IP addresses.

Enhancements

The readme contains instructions for updating .htaccess so as to also block bots that scan for and attempt various attacks on the site.
You can also block bots from voting if you have the Vote up down module installed.