I manage quite a few phpBB forums, WordPress blogs, and MediaWiki wikis which were (and still are) hit by spambots.
These spambots are almost always generated via an automated process. It's usually not someone making these accounts manually. A bot crawls around the web looking for sites which run on SMF/phpBB/vBulletin. Once it has found such a site, it attempts to register an account and if it can successfully get past the captcha, it either fills in the profile field with a link to a website (again, this is all automated), or it makes a thread/post containing the link, or more commonly it does both.
It does this in order to generate backlinks. The more other sites link to a particular site, the higher the site is ranked by Google and other search engines. An example of a program which does this is XRumer:
XRumer is a search engine optimization program, created by BotmasterLabs, that is able to register and post to forums (forum spam) with the aim of boosting search engine rankings. The program is able to bypass security techniques commonly used by many forums and blogs to deter automated spam, such as account registration, client detection, many forms of CAPTCHAs, and e-mail activation before posting. The program utilises SOCKS and HTTP proxies in an attempt to make it more difficult for administrators to block posts by source IP and features a proxy checking tool to verify the integrity and anonymity of the proxies used.
Link:
http://en.wikipedia.org/wiki/XRumerFortunately, I've found that these spambot-generated accounts tend to be easily distinguishable from genuine accounts so the risk of accidentally deleting a genuine account should be quite low.
