@HarmonicDisturbance to revoke all prior sessions, you should revoke the app from https://github.com/settings/applications.

There's also a ~10 session limit - so logging in several times will sometimes revoke past sessions.

There's no way to revoke a single session currently. I recommend revoking the app and logging in again or authenticating with a short-lived fine grained personal access token for your sandboxes.

It is a discussion topic to improve this, but those are the paths at the moment.