Simplest way to block a single IP?

[H.Wade Minter]

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I've got a system that's sending a ton of referral spam to websites on 
my RELENG_4_9 system.  I'd like to block them from accessing my system 
at the TCP level.  What's the best and easiest way to do this?

I assume I'll need to recompile the kernel with IPFIREWALL or IPFILTER 
support, then set up some rules.  Does anyone have a recommendation for 
a simple ruleset to block one particular IP?

Thanks,
Wade
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