Some people have a folkloric concept of how alerting systems work. Generally, a single event, such as one person's clicking the Report button on a spam post, doesn't trigger an alert. A minimum number of identical events is required, and after the threshold is reached, there's a "backoff" period during which the alert isn't repeated even if identical events keep coming in.
Despite the threshold, issues will still be too numerous for a human operator to handle, so they are presented in order of priority. It's more likely that a moderator sees that one post has been reported ten times, than that they see ten discrete reports for the same post.
We're not talking about Three Mile Island, where the 1970s dot matrix printer in the control room didn't finish printing warnings until days after the reactor event was over!
Posting a message to let others know that you've reported spam discourages others from posting in the thread, yes, but it also has two perverse effects. First, it discourages others from clicking Report. Thus, you undermine prioritization and let spam go unaddressed longer than necessary. Second, it alerts the bot responsible for the spam that a good target has been found. (Bots enable response notifications or check back periodically.) Thus, you train a bot to redouble its efforts against a Web site, returning from a new IP address, creating a new account, and posting more spam.
If a note to others were helpful, software would generate one automatically after you clicked Report, or add a symbol or color indicating that potential spam had been reported, or hide the original post pending review.
Instead, there's a request at the bottom of every page not to post responses to messages that violate forum guidelines. In the absence of further guidance, I'd follow the instructions given, click Report each time I saw potential spam, and not post to let others know that I had gotten there first.
I think people who want fake papers, etc. are at least smart enough not to respond to these advertisements publicly. I'm not worried that a lively discussion would ensue, keeping spam at the top of the new message list.