Most spam comes from scripts of hacked websites. If a provider hosts a few hundred domains on a single IP address, you may see his default web page, but the spam comes from one of the websites. Normally, such spam actions send some spam into honey pots of blacklists, too, so soon the IP will be blacklisted with major blacklists. No need to report it separately. If you want to, you can always send your complaint to the postmaster@ or abuse@ account of the domain or the network admin of the server.