I doubt we need additional motivation for people who otherwise don't care about the forum enough to report spam. Just like the sig-bounty "motivation" to create shitposts this would just cause new headaches. It's a recognition badge for people who are already motivated and let's leave it at that.
I didn't really think that motivation was the idea here. I thought it was about raising awareness. I know I used the forum for a long time before I ever thought about using the report button and I would think many others are the same. People that don't like spam are going to see that badge and that might be enough to make them think 'oh, I'm supposed to do that'.