Set up exim.conf to send such emails to a program you write, which will log them to a log file you create. Install APF (Advanced Policy Firewall) and set it up to watch that log file, and upon the first post to it, block the sending IP#. Personally, I think this is overkill; it's probably a greater server load then just blocking them.
The following works effectively but breaks a lot of RFCs and could conceivably get a lot of people mad at you: just create the mx records for these domains to something like null.example.com (your domain name there), and create an A record in the example.com zone pointing null.example.com to 127.0.0.1.
Very effective; anyone spamming you will be spamming their own box.
However it will eventually get you in trouble. Here's why:
You're presuming that all domains you're freeparking will have never existed before and therefore not have legitimate reasons to get email. That's just not a reasonable presumption. The first time a legitimate AOL user sends an email to a previously legitimate address at a domain now pointing to you, and their server gets blocked, AOL will block your server.
And they're not the only ones.
Frankly, I'd probably do it as well.
Try to be a good neighbor.
We also want to prevent spammers from using mail.freeparking.example.com and mail.abc.com to send their spam email. For example, if DA and / or our email server detects such attempts, we want them to recognise that as a network attack and block further connections from that sender in the same (5-minute) period.
Don't set up the mail subdomain for these domains. But don't expect it to matter; most spammers looking for open relays (and not too many do that any more) look by IP#. You'd do better by using your firewall to block port 25 for the IP# you've used for the freeparking site.
We also want to prevent spammers from sending their spam email with email addresses ending with freeparking.example.com and abc.com, if this is possible.
Actually, this is the easiest one of all.
Sit on them, so they can't reach their keyboards.
Of course to do that you have to find them.
Jeff