Yeah its definitely things like this that the murder skill are for. Heh.
And whilst the trees aren't your fault if you didn't cut them down, you were gonna get attacked eventually for putting your camp right on the water like that. It might be mostly impossible to avoid ppl with giant monitors with how close the rivers are together, but you can avoid most griefers just by getting a good ways away from the water. Yeah its inconvenient, but safety usually is. Granted if this was your neighbors as you say then it probably wouldn't have mattered, but it might keep the next group of griefers coming thru from stopping and taking notice after you hire some mercs to slaughter these fools.
All I can say is start making a buttload of bricks or getting a ton of cavebulbs and hope you can find you some hired guns. People say a lot to just get murder but the problem is the griefers are most likely higher level than you and they probably actually know how the combat system works (unless they're pure thieves not murderers as well) which means you would lose anyway unless you know as well. Its sorta convoluted unless you're using ranged.
Thats another suggestion; if they don't have good armor then you and your boys could trade for some good slings somewhere, along perhaps with some good specs, and just go group-gank them. I think you can keep knocking them out until they die without murder, and if you use the primo griefer tactic on these griefers (shooting from behind a rock near their wall) they can't even shoot back, they have to come out and get you by hand. Even if they can still wipe you and your guys out, you could have a guy stationed to go in and steal/wreck their shit as soon as they open the gate assuming they don't use signposts (which a lot of ppl don't). I'm not encouraging these tactics to grief, but if someone is already griefing you and you're sure its them then by all means show them that you can do it better.
