the OS in AOSP stands for open source.

They can't stop anyone from customizing Android outright, but they reserved the right to ban them from the Open Handset Alliance, a lá Amazon.

The closest I've seen to straight up bans from Google are against malware/virus apps and selective bans against violators of their ToS (some ad-blocking apps were banned for using a workaround, but other apps that used that workaround were not banned).

Don't forget that Windows is closed source.

While they could potentially close off the platform going forward, they can't retroactively un-open source code.