I have asked for this many times but with no luck.
I think I recall this suggestion before at least once. If I remember correctly, one of the mods--could have been hilariousandco--suggested that you can write something like "all of this member's posts are spam" or something like that in lieu of reporting all the offending posts individually.
Back when I was actively searching for shitposts to report in the pre-merit system days, I would go through members' post histories and report them one by one, and it was a pain in the butt. I'd love it if there were a check box function or something similar when you're reporting posts, because often members make a bunch of posts that should get reported all for the same reason. It's inefficient to report them individually.
I didn't know suchmoon made a script for this. I'd like to see it if you can provide a link.