Chuck Goolsbee wrote that one of his clients was having problems because miscreants have hijacked IP space that they own but haven't actively used in a while. While it's definitely worth submitting it to completewhois and developing whatever paper trail it takes to give it back to the registrars if they don't want to keep it, another obvious stopgap would be to advertise the space, including their /21 and any /24s they see route advertisements for. Either point it to some spare PC with a web server handing out "Forgers hijacked our address space" pages, or null route it. Also check the reverse DNS listings, if there are any, and have them advertise a pointer to a subdomain like weve-been-hijacked.theirdomain.com with an appropriate web page.