Bukkit

Since you say youre not able to give them the ability to defend themselves, what about making a villager with a job invulnerable?

I already have the players pay gold when they kill one... but with griefers around, they wont care about having no/minus gold. :(

Perhaps as an admin configuration/command.

but I'm not very fond of this sort of thing (invulnerability). I'd prefer use priest that set on fire any villager-killer nearby, possibly use the iron golem as a keeper that keep the mean players in mind to beat them as the come too close.

Of course as an admin-option. One Idea is to make it in the config. Other Idea is to punch them with a golden apple or something to make them invulnerable.

@fablecraft: Go

The golden apple combined with permissions and/or admin commands is a good idea. Thx.

Let's push this in the pipe.