All mods will help prevent them from entering the site but cannot stop them from trying. The Forum Firewall may help with that but it is very easy to make the wrong setting and ban yourself or others or even disable your forum unless you know what you are doing!
The best way to stop them from hammering the site is to use a .htaccess file or use cPanel's IP Deny feature. You can set a range or IP's in either of those if, let's say, 20 of them have the same IP but the last part (host) is different like 111.222.333.856, 111.222.333.482, etc. You can use "111.222.333." or "111.222.333.*" depending on how your hosting is set up. This will block the entire range for that part (the last part) of the IP...numbers 0 through 999.