Are you certain the mods cannot block specific IPs? That's an extremely basic feature of every forum software product I've ever seen (and I've seen a bunch). I gotta think it's a feature of XenForo.
Actually I'm not sure, so I'm hoping a mod can chime in to confirm or deny. I only glanced at the thread complaining about the weirdo harassing people and remember a mod admitting they were pretty much powerless.