Answer
The easiest way to ban mail from a specific IP address is to use the banned host option under GMS Anti-Spam. You will need to add an entry with the value of * and any IP addresses that you would like to ban should be listed as !
e.g.
* !123.123.123.123
If you do not have GMS Anti-Spam then you can setup and Incoming->Redirect rule
which will reject the remote IP address something like:
Mail Clause: * Remote IP: 123.123.123.123 RCPT Clause: * Local IP: * Action: Protocol refuse with... Parameter: Error message.
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