|Chances are if they're spoofing their IP's in order to keep coming back after you've banned their IP, you can stop them by putting a ban on the ISP's full network.|
So, if the ISP's Subnet is a class C you would ban xxx.xxx.xxx.*
If a Class B: xxx.xxx.*.*
If Class A: xxx.*.*.*
Alternatively, if you're logging the sessions, contact their ISP and report them.
Where is the server physically located and what chat software are you running?
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.