If you are an administrator, it would be advisable, to check the configuration of your php application. If you [u]allow anonymous posting[/u], your forums could end filled with spam.

Some of the program can register account automatically, and add their web site link as part of account information. Since most forum will display link for new register user or profile for all users, it reach the goal for link-building purpose. [b]Only allow user profile to be viewed by registered user[/b].

Although requiring a user to register is a good idea, some bots are able to do so.
There are "tricks" that administrators should use to prevent bots from registering in the forums.
Use security plugins:
1. Add Visual verfication image.
2. There is also a mod called "The Humanizer" which simply adds another question to the registration process. In particular "Are you a human being?" and two possible answers yes or no. Although this may sound quite obvious, it works. A general purpose bot, will not be prepared for these small modifications.
3. Require user to activate the account.

However, some program can bypass the above procedure. Like Xrumer or Zunker...
Please chout out  [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1016.0]How to block some programs which can register and confirm account automatically!