Although I would only send one report per spammer if a saw multiple posts or threads from one, it can be helpful to report all such posts. Speaking from experience here.
It's always an option to simply report the spammer's profile, too.
If more than one person reports a specific post or user profile, all of the reports go in the same report "thread", so commenting like you described is unnecessary and only adds one more post that needs to be removed (although, if it the spam is in a separate thread as the OP, then deleting the spam takes care of the entire thread, including any comments).
There is a spam cleaner tool that allows everything a spammer posted to be deleted in one go, and by default it also permabans the spammer plus auto-resolves any reports about them. It has options for not doing all of the possible actions, too.
I would assume that tool is in use here, and if it is not I would absolutely recommend it. Saves a lot of time.
(I am very familiar with how reporting and spam removal works on a XenForo forum like this one.)
Edit: In the spoilers below is a quick example of what a report handler sees when opening a report about an item that has been reported multiple times. It is all on a single page, and one action resolves the entire "thread" of reports.