1: Go here:
Code:
ACP > Configuration > Banning.
From there, enter all the spam bot IP's that has registered in your forums so far and block them. Disallowing email address won't work cause they will just create a new email address thus, you will need to ban them from there IP address. Then they will not be able to register.
2: Reduce your registration to 1 per account. Here is how it is done:
Code:
ACP > Configuration > User Registrations & Profile Option > Maximum Registrations Per IP Address :
Change that to 1.
3: Also change your registration time for each IP: Time Between Registrations. Change that to like 1000000 meaning they will be have to wait that many house before registering a second account. That should reduce your spam bots.
4: Enabling the add Captha in your forum so when a human is registering, he will know how to type it but when a bot is registering, he will get stuck there. But be aware, modern spam bots have the ability to pass that section.
5: Deny IP Ranges with .htaccess
This is fairly complicated and if you don't know what an .htaccess file is then you should either learn before preceding or just skip this option and go for a more simple approach stated above. A basic .htaccess ban command would be something like deny from 222.35.1.4/30 that will not allow users from the ip range 222.35.1.4-222.35.1.30 to access your web page.
*I did not write this, I just found it and thought it would be helpfull.