(Personally, I would consider them spam.)
I was astonished that they didn't do this when they introduced the emoji reactions feature.
I think it could be done on a per repository basis, like, basic behaviour rules just like at forums and other such communities. It's then up to the maintainer / moderators / whatever to allow or disallow +1-style comments, and what the 'punishment' would be. They could then also tweak a setting that auto hides comments if they're shorter than X characters.
I also think they're still valuable when you have Github and another VCS host, where one mirrors the other automatically.
Well then the user would learn nothing. A better solution would be to notify them that this is wrong, delete their comment, and then prevent them from commenting on github.com for a period of time (1 hour? or more?)