Have you seen someone commenting on your site like this?
Users often comment on sites and leave a backlink to their site in a spammy manner.
As seen in the image above, such links created by users not only degrade the user experience on the host site but also reduces the rankings and traffic.
Now, have a look at this forum spam:
Forums are a great source of information and hence should not be buried by auto-generated keywords and links placed there by spammers.
And remember, your site can be penalized by Google for having too much of user generated spam. The below video explains what it means if your site has a manual action for "User-generated spam" and what you can do to fix it.
Here are some great ways to protect your site from spam:
1- Keep the forum software updated because older versions are more vulnerable to spam.
2- Use a CAPTCHA. These require users to confirm that they are not robots. Try Google reCAPTCHA and Kitten CAPTCHA.
3- Block excessive traffic coming from individual IP addresses.
4- Set time limits between individual posts.
5- Apply rel=”nofollow” to user-generated links.
6- Check regularly and remove spam content from your site.
7- Do not allow users to post blogs automatically. Set them to moderation or add rel=”nofollow” attribute to the links so that the passing of page rank can be prevented.
8- Have some restrictions for new users and allow them full access only after they reach a certain level.
9- Use automatic spam filter services like Akismet.
10- Blacklist domains hosting multiple spammy profiles.
Google Penguin Recovery Guide
How to rank in Google
Are You Making Your Website Vulnerable To Future Google Updates?
Google Disavow Links Tool