SMP, as I understand it, currently functions a lot like an IRC channel. So you have ops who can kick/ban people, but everyone else is equal in terms of power, and there may not always be an op awake and active on the server.
Perhaps it would be appropriate to have a "police" role where you can incarcerate/disable alleged griefers until an op can deal with them. Only users trusted by ops would be police, of course. I hear some servers already use "spawn jails," but those are proactive and a little rude (in my opinion.)
On SMP, does everyone spawn in the same location? Once PvP is available, and if it's totally unfettered, I could see griefing on PvP servers becoming very short-lived. You'd have to stage a substantial "invasion" if you wanted to be able to do much damage.