That's why there are inbound controls on the Mail config document

HCL Notes/Domino 8.5 Forum (includes Notes Traveler)

Subject:

You need to get the IP Addresses from your spam gateway and only allow connections from there...

Inbound Connection Controls
Verify connecting hostname in DNS:	Disabled
Allow connections only from the following SMTP internet hostnames/IP addresses:
Deny connections from the following SMTP internet hostnames/IP addresses:
Error limit before connection is terminated:

But if you really wan to stop them hitting your server use a firewall and block the traffic even getting to the Domino server unless it is the trusted IP

Feedback response number CTYR9HKJC5 created by ~Mark Bubnitexader on 03/26/2014