Towny

Now I haven't actually joined your server yet so i don't really know much about it but here is a suggestion:

Install the mod known as Towny as it would allow players to control anti-grief by letting them define their own towns AND allow them to actually buy real plots off the admins which would in turn allow nations to form and eventually inter-town and international war which i think would be kind of kickass.

If you need any help installing this or if you don't even use the admin mod "Bukkit" swing me a mail [mod-edit] as i have had experience in setting up about 8 minecraft servers with multiple mods(Mostly Bukkit though with a ton of plugins)