I'll try to throw in my two cents.
If the two players had homes near the quarry, then I could understand aggravation at a massive hole in the landscape, and could understand rage clouding common sense. However, by the wording in the OP, the two players set up shop IN the quarry after it started, having a merry old time.
Personally, I'd step back and let anarchy fall upon the server by giving the victim permission to retaliate, but I can understand that isn't really an option on some servers, especially this one. Total war is fun, but only in organized events.
As it stands, I wouldn't ban the perpetrators either, as they have to atone for their crimes, for that is what I see it as. They willfully rendered the work of one of the members useless. They might as well have stuck a filter pipe on the quarry and left. Not cool.
Nothing less of setting them in adventure mode and keeping them locked in a player jail for a month or so as a warning would be satisfactory in my book. If discipline is to be administered, you might as well have fun while doing it.
It should discourage repeat incidents, and also mollify any others who had sneaky ideas.
RUURRUUS!