Ok, so it was less than two weeks ago, after the new rule was in place.
Here's the thing Kart - I don't see much extraordinary damage. They took the brick walls because brick is valuable. They took the chests because they can be used elsewhere. They broke some of the stone or cobble walls because they were looking for hidden chests. The lava (and water, if it wasn't already there) is the ONLY thing that could be considered breaking the rules. Well, if the front wall wasn't brick (say, it was sand), and was only one block thick, then they shouldn't have broken that either. So possibly two grief incidents, but I don't know what the house looked like beforehand. I agree that it would warrant more investigation, but I just have no further information available, and no more investigation that CAN be done.
We can't catch griefers every time. Most of the time, we can (and do), but sometimes circumstances present themselves in such a way as to be impossible to identify with certainty who the griefer was. Of course, no one who listed their names on those signs will admit to placing lava, especially after reading through this discussion, so who should I ban? I can't ban all of them, because many of them were most certainly not responsible for the lava, and I can't ban the person resposible, because I don't know who it was.
I can't even say for certain that it was one of the players who raided the house. People do strange things with lava - they'll put it out in the middle of nowhere just because they can. Maybe a guy found the house, and was mad that it had already been raided, so he put a couple of buckets down just to make the houseowner mad? You might not buy it, but not buying it is a far cry from proof that it wasn't someone else.
I'm locking this thread, but feel free to continue discussion of this with me via PM, along with any concerns you have. The public discussion here is done.