K2 is probably the most popular Joomla component today. It enables Joomla with two very important features that per our opinion should be incorporated in the core Joomla. Beside the many other features, those two most important features are content tagging and commants of articles. The comment form in your article pages is one of the most often used for spamming. There's no better way for a spammer to get a link from your site than to leave a pointless comment on your article.
Is your Joomla website hacked recently? Is this repeating from time to time? Did your web hosting provider warn you that your account may be suspended if this repeats again?
Even if all your answers are "no", it's still recommended to follow these security steps to make your Joomla website secure.