I'm kinda surprised this wasn't already a thing - I've never seen a "buy viagra here" comment on GitHub so I assume they have had some anti spam system in place for ages.
I hope it's not going to turn into another weapon in online political battles.
It's used by humans. It will. Hopefully, such abuse will be characterised as just that, but it probably won't be because humans will judge the reports.
I think the reason you don't often see spam on GitHub is also a matter of convenience. While GitHub as a whole is very large, there are few people reading a specific issue, and posting spam elsewhere doesn't really make sense (nobody will look at your "buy-viagra-now" repo, nor will many people notice a new issue/PR called "Buy Viagra now"). And even then, you can assume that all readers are sufficiently tech-aware and won't fall for direct spam. From the spammers' point of view, you're wasting resources on a platform that has virtually zero chances of yielding visitors, as opposed to mail spamming which has higher chances in virtue of higher visibility and less conscious users.
You could report users beforehand and I think leave links to specific examples in the comment field. And there has to be some kind of automatic system as well.