If you are in the grist-core repository, you can do something like:
$ yarn cli --help
$ yarn cli history prune docs/gy4qk3Cd3YoNquH6ZjJmDo.grist
Otherwise, there is an undocumented endpoint /api/docs/<docId>/states/remove:
You can POST to this with a keep parameter of 1 (say) to remove all but the last action from history.
The action history will eventually be truncated once it is “big enough”. This area definitely could do with some love and UI options.