I'm afraid you're probably SOL.
Banning IP addresses only really works if the IP address is static, or if it's dynamic but seldom changes (as with residential cable modem or DSL service). With dialup, the user gets assigned whatever IP address is available from a pool when the connection is established. So you can ban the domain, or ban a range of IP addresses, but you run the risk of throwing the baby out with the bath water.
That said, I hope you or the board mod can figure out something to do about the troublemaker.
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