It would be good if there was a dedicated option for this.
However, in previous versions of Plesk all you have to do is create a domain, and set it as the default domain on your IP. This domain will then be displayed if an account is switched off. Maybe it will work with Plesk 9 too?
As to getting a "different website" -- do you mean a random one? If so then that kind of indicates that you might have more than one PRT record per IP, which is not a good thing. This is just a guess, but anytime you get random domains it is a reasonable guess.
There should only be one PTR per IP, and normally it should be for the main domain on your server. Indeed, in this case, the default domain mentioned above.
Sorry if this isn't what you meant - just ignore me.
Faris.