Oh, actually you can. After all, if there are companies constantly indexing the whole www, or even making a "backup" of it, why wouldn't be possible to index information about domains and IPs?
There are some free services that do just that, like for example:
And there are probably many others that work in a similar way: you provide a public IP address, and you get a list with the domain names that resolve to it.
This can be quite useful for several purposes. Tracking a spammer seems to be one of them.
I don't know how accurate, current or complete is the information that these services provide, but they have helped me in dealing with situations like the one you described.
As a final note, I suggest that you take this particular question to ServerFault, where I'm pretty sure you'll get more and better answers.