Put 'em on their own IP number. That's how it works after all. rDNS doesn't tell you every site hosted on the same server. Just the same IP.
<edit to add>
Just in case anybody thinks to try these rDNS tools available out there to do some snooping on their competitors, realize first that because two sites sit on the same IP number doesn't mean they're owned/run by the same person. There is tons of shared hosting out there these days.
Also, do not place full and immediate trust in the returns you may get from some of these tools. Too many of them cache data and never double check what they tell you. A good for instance is Google.com. Using one of these tools (not the one Chris mentioned, I've never tested it) if I do a rDNS lookup on Google.com it gives back three IP numbers, showing that each IP number has multiple domains sitting on it. To the tune of some 1,800 or so domains supposedly sitting on the same IP number(s) as Google.com.
But if I do a manual, real time rDNS lookup via Dig to spot check some of those domains I couldn't find a single one of the dozen or so I checked that was actually hosted on the same IP number as Google.com. So a few grains of salt is definitely in order sometimes. FWIW, I get the same kind of shady results if I plug in one of my own server IP numbers where I can see exactly what domains are hosted on the IP number in question. Where the number of domains returned is usually somewhere in the right neighborhood, but not right. and the domains the tool tells me are hosted on my server are all wrong.
Be careful with those tools until you've proven to yourself you can trust it's output. It's easy to do with Dig or even nslookup if you have a few spare moments.
Edited by Randy, 08 August 2008 - 09:20 PM.