Unfortunately, there are spam bots, programs that automatically detect web posts that are open for comments, and put in spam comments into them. The pain of having these comments coming in unmonitored is that your site loses it’s reputation or online reputation as it is called today, and also, if monitored, could lead to excessive time to reject or delete it.
One method to keep spam bots away is to enable moderation of comments. This gives you one line of defense to log in to your blog and delete or reject the spam. The second line could be the use of a plugin called akismet. However, end of day, it is up to you to kill the spam manually. Another method is to use a Captcha. This ensures that those who leave comments on your word press site enter some text or solve a small math problem to prove they are human. Some spam bots do overcome the simpler captcha plugins.
To make it more difficult for spam bots to post comments, you can use Blue Captcha wordpress plugin. This throws a random graphic with random letters & numbers which are difficult for automated spam bots to recognize. Reduces spam comments by almost 100%. Also, the statistics on spam origination, content appear on the dashboard for you to decipher and act upon.
I earlier used a Captcha plugin that generated mathematical formulae for a person to solve. It would display numbers and signs in either text or digits for a person to read and solve. Unfortunately, that restricted the space used and was ideal for a spam bot to work over.
So, next time you ask the public to give comments to your wordpress posts, make sure you use a good Captcha plugin like Blue Captcha.