[Exim] Spam emails problem with mail server

Hi all,

I have Exim 4.30 with latest exiscan patch in FreeBSD 5.2-current machine.
I have also SpamAssassin and black
list ckeckings in exim config.
I'm running free web mail service on this machine. Everything is working
fine except some
people are sending thousands of spam emails to the Internet from web interface.
How to prevent sending thousands of spam emails? How to check it?
I tried to block accounts but they just created new accounts and start sending.
How to block IP addresses? Will it solve the problem?
What happen if they change IP addresses since there are many open proxies
around?

A I said before I'm checking black lists at SMTP time. Since spammers are
using web interface it is not checking.
How to check IP addresses when they using web interface? Are there any such
methods?

thanks in advance,

Ganbold