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
Here is an example on how the SpamDog work
This test will check the IP address against several DNS based blacklists. (Commonly called Realtime blacklist, DNSBL or RBL). If your ip has been blacklisted, some email/comment/registration you send may not be delivered. Blacklists are a common way of reducing spam.