It's so users don't abuse the system or (mistakenly) report something multiple times.
When you report, it sends a notification to the staff who then take the appropriate action. In many situations they even look further into the user/post to discover is this is their behavior is frequent or just an oddball occurrence.
That being said, when a bot spams the forum, you really only need to report 1 or at most 2 posts. From there, the staff will see that the user is spamming the forum and take actions against all posts deemed to be spam.
It's a very common practice used on many forums. Keep in mind that the report system isn't an automatic deletion/hide feature, but rather it alerts the staff to check the reported post if not the entire thread. Even if 50 users all report the same post, nothing different happens. Again, this is common practice, practiced on many many many forums. Larger more mainstream forums with million-plus users are more automated, but on more private forums such as this, it's not needed.
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