It doesn't seem to be a method to unregister a user from push notifications.
My use case is a single user app with Facebook and Google logins.
A user installs the app on their Android device, and logs in. This creates a new user, with a Facebook account and an AndroidDevice linked. Also, registers it for push notifications.
The user uninstalls the app and gives the Android device to another person. The other person installs the app again, and registers using Facebook.
I can't assume that the same AndroidDevice is the same user, because those were different Facebook accounts. So I create a new user and link the Facebook account, and link the AndroidDevice, and of course it registers for push notifications.
When using LinkAndroidDeviceIDRequest with the ForceLink parameter set to true, the first user is unlinked, but the push notifications information remains, so the same device will receive push notifications for both users.
This is pretty unconvenient. And I can't seem to find a way to solve it with the current API.