The obvious answer is to end free unlimited private repos. Source is no longer a specific thing. GitHub is just a free S3 or Azure provider.
I think this would cut out a lot of the mindless commit activity.
Charge $1/month per repo. If your project isn't worth that, run your own git or use another free forge.
I would have thought it's public repos that got the most AI slop recently as a form of gathering karma and boost CVs. It would be nice to know if private repo commits increased instead.
A lot of private repos are probably just taking up cheap storage space, not seeing activity that would cause additional load.