SpamCop reports include a URL that allows you to register an issue (IP/datestamp or website) as "resolved." For a website, you even have the option of registering as an "innocent bystander." In either case, anyone who tries to report the same issue through SpamCop in the future will receive a message stating what action you have taken and they will be prevented from filing a report on the same issue. This keeps your workload to a minimum and lets spam fighters know you are helping wipe out spam!
See also the next question related to responding by email instead of the web.