The problem is that no matter how you approach it, griefing is "game content" for some people, and they will do it. It does not matter much how you try to tackle the problem:

Not really the case. The problem is the greifers are able to get something for their time invested in griefing.

Griefing does not have a need of "getting something" (loot, kills). Griefing is about causing grief to people, and this is (at the moment it's done) the game's main content. The same people may be "normal players" at other times, usually with a different character name.

If one "gets something" in addition, that is probably a plus, but the main incentive is to spoil another person's day (presumably because you just came home from a lousy day at Burger World and hate your life... or your imaginary girlfriend left you... or whatever... maybe just because you're bored, or... think of a reason).

Real examples of griefing that I've seen include people standing in doorways to shops or save points in games with collision checking and without player killing (or very harsh penalties on PK) for hours, until removed by game masters, just to deny people entering.

Or, people buying out someone's craft goods at the merchant, or intentionally ruining resource gathering spots -- both used to be quite common in Ryzom at some time. I remember people making newbie characters for the sole purpose of joining another guild and sabotaging them by taking away their time and resources. I also remember people declaring war every night at 5:30 in the morning for two weeks (and not showing, except maybe once).

Or, of course, aggro dragging. Needless to elaborate.

Tapping elites/bosses with an instant warrior shout attack quickly just before someone else hits them was done a lot in the early days of LOTRO, both for stealing kills and for annoying people into leaving.

Griefing can be anything, and it doesn't even matter if it costs you something, as long as it annoys the hell out of someone. And that, sadly, is just how some people play, and it's something you cannot easily combat.

Left 4 Dead had no time for Griefers.

Though Left 4 Dead is not truly a RPG.

Demon and Dark Souls have great communities. Griefers are usually power gamers too.

True on both accounts. As I've said above, community can help. In fact, a good community is probably the most effective countermeasure. Anything you do rule-wise only hurts players, not griefers. Griefers don't play by the rules.

Though even with a good, strong community, a griefer may just use a second character unbeknownst to the others for griefing. Gaining souls (or money/whatever) doesn't matter to a griefer.

A griefer just wants to piss off people. A griefer may be doing some-sort-of PvP (not something a "real" PvPer would deem PvP though), but needs not.