you're not alone, over the past month, I bet we've had a dozen users in our company get slammed. It's only for one day though, actually more like a 5-8 hour period, and the ndr's stop.
It's a pain, but like xmsre said, really nothing you can do other than explain it over and over again to the user of the day it's affecting. Eventually, ISP's will probably get caught up on this new ndr spam craze and will find a way to put a good dent in it.
friggin spammers anyway..