Yep, SEO the shit out of the sites, in the most transparent, sure-to-get-a-site-delisted ways possible. Internet Judo, use his strength against him. Since they're on top, it's not like you can make it any worse, and it also means you don't have any direct contact with the guy--it's Google/Bing delisting him, not you. Anonymize the WHOIS information, and deny any knowledge if the guy contacts you. "What, you put up a site saying bad complaining about me? Weird, man, but whatever floats your boat. Good luck with that I guess."
And of course if Google/Bing contact you, just say, "I've been contracted to make this the top result, if you try to delist this site you'll be hearing from my lawyers." They already know you can't do jack, and they'll enjoy tweaking "your" nose and you'll find the site delisted in short order.
Other alternatives
DDOS: Illegal, don't be an idiot. Also feeds the troll, you know better than that don't you?
Register trademark & use ICANN: You're rolling the dice here, feeling lucky?
Anything + lawyer: Probably best chance of success, once you sue successfully it just makes further suits easier if he's dumb enough to stick a fork in the toaster a second time. But keep in mind, these guys do not understand the Streisand effect, and what's more, many probably actively want it. Free publicity for them, and then you have to pay!
Completely ignore it: Probably the best option. People get bored. He's doing this to get a rise out of someone who rejected him. Chances are good he'll escalate when he doesn't get the reaction he wants. If he goes big enough, you'll be able to catch his hand in the bear trap of the court system, otherwise just keep ignoring it. Escalation means you're winning. If he's quietly running the same site 3 years later, well, then you're dealing with a patient, smart, asshole, which is pretty much your worst nightmare. Good luck.