@Jude19, now I think I've seen it all. They cannot be serious, blaming you for this. Perhaps they didn't realise the low water pressure had to do with the volcano. But given that they did contact you to complain about the water shortage, when presumably you would have explained the reason, the review they left is inappropriate and just mean-spirited. And it is the height of rudeness to not even acknowledge your rebate. These people do not deserve to be Airbnb guests, full stop. I just wish you'd left them a negative review so they could not impose their bad behaviour on other hosts.
Anyway, I would totally ring Airbnb and ask them to remove the review on the grounds that the situation was out of your control. You might have some luck. Otherwise, I think your public response to the review definitely puts it in perspective for other guests - I would be able to see it for what it is. Best of luck and let us know how you go.