Spammers can be a forum problem. Here is what Wikipedia says about forum spam.
Forum spam is the creating of messages that are advertisements, abusive, or otherwise unwanted on Internet forums. It is generally done by automated spambots.
Types of spam
Most forum spam consists of links to external sites, with the dual goals of increasing search engine visibility in highly competitive areas such as weight loss, pharmaceuticals, gambling, pornography, real estate or loans, and generating more traffic for these commercial websites. Some of these links contain code to track the spambot's identity if a sale goes through, when the spammer behind the spambot works on commission.
Spam posts may contain anything from a single link, to dozens of links. Text content is minimal, usually innocuous and unrelated to the forum's topic, or in a very old thread that is revived by the spammer solely for the purpose of spamming links. Some text is included to prevent the post being caught by automated spam filters that prevent posts which consist solely of external links from being submitted. Full banner advertisements have also been reported.[by whom?]
Alternately, the spam links are posted in the user's signature, in which case the spambot will never post. The link sits quietly in the signature field, where it is more likely to be harvested by search engine spiders than discovered by forum administrators and moderators.
Recently, a very destructive forum spam attack has been propagated by inserting into comments redirect domains with an automated posting script like Xrumer. These domains redirect a user to pornographic Websites. If a user clicks on the image or attempts to close the Website an ActiveX codec will be downloaded as a Zlob Trojan.
Effects of spam
Spam prevention and deletions measurably increase the workload of forum administrators and moderators. The amount of time and resources spent keeping a forum spam free contributes significantly to labour cost, and the skill required in the running of a public forum. Marginally profitable or smaller forums may be permanently closed by administrators. Forums that do not require registration are becoming rare.
Be smart about what links you click on. If a link is either posted or private messaged to you by a poster who you don't recognize, has no other forum posts and has nothing to do with the subject of the forum, DO NOT CLICK IT! If it is a forum post, report it to the moderator. If it is a private message, post about it here. You don't need to report it to me, because I will have gotten it, too.
So be on your toes about spam and remember to report any problems so it can be eliminated ASAP.