WordPress Simple Firewall is truly a 'must have' plugin for all websites. Helps a lot!!
WP Simple Firewall, which might be simple to use, but is actually very comprehensive and includes many useful options. WP Simple Firewall has a great way to block bots from posting spam and also requires that even "humans" .. wait a certain period of time (such as 5 seconds, 10 seconds, etc.) before they can submit a comment, .. and it can also require that the visitors checks off a specific box to indicate that they are in-fact a real person.
It has many other features as well. I use WP Simple Firewall on every one of my websites (in addition to other security measures that I also implement), and after adding WP Simple Firewall .. everything is just perfect.
The developer is also very responsive and helpful.
Gary Gordon (wpbns.com)