The most appropriate response to comment spam depends on its source.
If it originates from an anonymous comment, simply delete the comment and mark it as spam. This will forward it to a queue to be analysed, and an administrator will take steps to block the originating IP.
If the spam originates from another logged-in user, please file an Abuse report [ http://www.insanejournal.com/support/submit.bml ] so that the account-holder can be warned that his or her account has been hijacked, or so that the account may be suspended, if necessary.