I recommend you to use Better captcha from here:
http://www.phpbb.com/community/viewtopic.php?t=473222
one more is here: http://www.phpbb.com/community/viewtopic.p...p;hilit=captcha

After installing the same, the spam bot registration on my forum has been reduced ( No its totally solved : )
You can see my custom captcha image here in my forum registration page:
http://www.fun.niranvv.com/profile.php?mode=register

You can change the background properties of the captcha images and many more custom options will be there on admin control panel!
You can use some new fonts and update the captcha system regularly!


Niru

you're using phpbb that means it's open to hackers and spammers try vbulletin almost no spam. but anyhow. to i would install the following mods.

http://sourceforge.net/project/showfiles.php?group_id=136984
EasyMOD is a script for phpBB that installs mods automatically on your phpBB installation. it basically will make it easy to install any mod you just pop them into easymod and they install automatically.

http://www.phpbb-security.com/check.php
This tool will check your phpBB forum for the risk of being spammed. It will NOT check the phpBB version or perform general security checks! In order to stay secure, please be sure to always have the latest version of phpBB and all mods!

In order to perform the anti-spam check, our tool may try to post automated messages to your forum. After the check is finished, you can remove these messages, although we advise to leave the links to our site, as this tool may be useful to others too.

Depending on the speed of your server the check may take between 10 seconds and 10 minutes. During this time you won't be able to see the check progress. Please, be patient! Enter your email in the form below to have a copy of the report delivered to you as soon as the check finishes.

Thats the same thing with my forum site althoughth iam new but i used a differnt Forum hoster well i just postto say that Spam is overloading all forums

I'd have enough with these spammers. I was creating ease of use for my visitors to drop support requests or improvements they want to suggest without registering. However, the spammers are abusing that, so on my forums (powered by Quicksilver Forums) is now registered-only users can post.

xboxrulz

First to avoid the spam you have to of changing of phpBB to a much better forum as Vbulletin or SMF this is it better and that in addition have few bugs, and possesses small systems to prevent it from taking placespam, since to add a small image in the system register. The phpBB are good but they are very insecure, like that that my recommendations that you were changing.

Download some Spam Prodection