griefing in faction world, last i heard that was being debated whether it would be allowed or not.
if griefing in faction world is considered fair game, then I suggest you find his faction and grief him right back. but I dunno if thats the rule or not. if you look at /rules it still says faction world is questionable for griefing
also understand that there are ways to protect your base from griefers, since blocks cannot be broken unless they are in your faction, make sure not to invite spies or untrustworthy allies into your faction, and be sure to build within the allotted space for your faction... if you build on the edge or across the edge of the zone that is protected, then it gives access to whoever manages to find your vulnerable faction edge... so keep all your doors closed, build in your protected zone, and then all griefers can do is grief right outside your base, but not inside it.