First or later any website administrator have to combat this war, especially when your site starts to be a little bit more popular. Tipically the most common spam target at your site are:
This is one of the most common target of Spam that you might see on a typical Joomla site. I have seen tons of fake accounts this happening both by automated scripts as well as human
If you have a forum on your site you are a target for this kind of Spam. This typically contains ads text and links.
If you have a comments system to allow users to post comments on articles you are a target for this kind of Spam. This typically contains ads text and links.
If you use Contact the email account are a target for kind of Spam
The SpamDog give you some weapon to fight Spam:
Whenever a content or registration is sumbitted is first evaluated by specialized webservices like:ProjectHoneypot, StopForumSpam, FSpamlist, BotScout, Akismet, Mollom, Defensio, and some RBL
and only after an "ham" classification are allowed.
These webservices databases are updated continuosly so maybe the spammer could be trapped some days later