I don't go into the server, do people actively attack others now?
But anyway, I'm not talking about people using dynmap to attack others, but attacking claimed land and factions themselves, which can be done with TNT.
Another idea is toggleable PvP, where those who enable it can fight others, and those who don't care for it can just set theirs to off. I actually do like a little PvP to spice up playing with others. Pretty sure there should be a plugin like that, I played in such a server years ago.
Also, as long as factions/towns/plots or whatever are made invulnerable and applied from the map's start (unlike previous where Valhalla got unprotected when towny>griefprev), I don't see the harm of dynmap. Sure, griefers could still destroy unprotected land but they still totally can now.
EDIT: Thinking about it, I have some thoughts about how this server is right now, and some suggestions for its future. It's just my opinion, so you don't have to agree or disagree. You can even disregard it, since honestly, I don't go in the server much.
My 2c and suggestionsAs a guy who watches letsplayers pretty often, I've thought that MC servers could be generalised into a few types: Enforced-Order, Trust-Order, and Anarchy. Enforced-Order would kinda be like hardcore grief-prevention plugins like foolproof area protection, LogBlock, etc to ensure that people's things are protected. Trust-Order is basically a vanilla server, but a tightly-knit community (whitelisted, etc) ensures that everyone is friendly with each other and nobody actively griefs someone else. Anarchy is... well, anarchy. Do whatever the fuck you want to.
Most players here would prefer the Order side of things, as far as I can tell. Obviously, osu is nowhere near small enough to be a true tight-knit community, which means players want some sort of plugin, etc to help protection. I've never really understood the choice of Factions as an area-protecting plugin, when it's obviously not meant to be so. Claims can be taken over via death or removed due to inactivity, or completely skipped if someone has a large-enough supply of TNT. This is supposed to promote user interaction and some anarchy iirc, but the problem is no one wants to interact like that. So as a result, Factions is being exploited by griefers instead.
1.7's coming up, and the massive biome changes will probably necessitate a new map when it's released. My suggestion is that an alternative claiming plugin is used, similar to Towny or GriefPrev or something, idk. The plugin's claims would not be able to be taken over nor destroyed. For decay, it might be better to just leave inactive claims alone unless it's an area close to spawn and someone else would like to build there.
As for PvP, like I said, some people might like having it since it gives another thing for them to do. I myself like some PvP, since it can be deeper than just spamming, with the right equipment. A nice compromise would simply be a personal PvP toggle and also some method to see who else has their PvP toggled. Small rewards could be given to winners, like maybe the player head or something to hang up in homes. If Dynmap was enabled, people should be able to hide themselves from the map too, if they don't want to be disturbed or are trying for an ambush. It also makes PvP more interesting if both the attacker and defender are hiding.
For griefing of unprotected land, there's no way to actually prevent that, unless some chunk-restore plugin is used or something, which doesn't revert ores, for obvious reasons.
Of course, there's a completely different idea. I remember Eph talking about an idea of a limited map, and limited factions. Important resources will decrease over time, and players can either resort to using renewable resources, or use an economy possibly based on fighting and exploring outside of the faction. Factions can fight to expand, but each faction will always have an unconquerable "capital" as a safezone for those who aren't interested in fighting. Limited factions and safezones would definitely increase interaction between players, if that's the target of the server.