Like LoyceV stated I've also had conversations with different moderators on how to handle reporting a large amount of posts. I've come to the conclusion that it depends on the moderator as well as the type of post you are reporting. I know that when reporting bumps it's best to just report the earliest one in the thread and specify that there's multiple bumps in the same thread. If it's the same user posting the same nonsense then you can just report one of their posts and ask whoever is reviewing the report to take a look at their post history for similar posts.

Although, I won't mention the names of the moderators, because at the end of the day it's up to them if they want to specify how they would prefer to handle things. I report individual posts in long threads if they are posted by different people and know the moderator of that board prefers that all of the posts should be reported to make it easier for them.

Of course, it isn't always that the moderator of that board will be dealing with the reports, and the likes of hilariousandco and other global moderators/admins deal with it instead. But, after a while you get used to the timezones of these people and know when to report different. Hilariousandco probably sent you that message specifically because, it was all made by the same user.

The rule of thumb that I usually follow is if the same user is posting the same garbage just report one of their posts and note in the comment field that there's many more like it in their post history. Sometimes it's easier to identify situations such as posting "res" type posts in small threads you can just report one of them and let the moderators know in the comment field that there's more like it on the thread from different users.

In your example in the OP; It would likely be sufficient enough to report one of their posts and specify that there's many more like it in different boards. The moderator who deals with this can either act on it themselves if they have the permissions or leave it for someone who does have permissions to deal with all of their posts and whether any other action should be taken.

Just think of it this way; If you can make the person reviewing the reports life easier by specifying certain things such as taking a look at the users post history, do so instead of reporting all of them.