How to stop deployment to Heroku in progress

First, install the Heroku builds plugin:

heroku plugins:install heroku-builds

Then, to cancel a build, fetch the list of the recent builds:

heroku builds -a YOUR_APP_NAME

The first line of the output will be your currently running build, the first column is the build ID.

Cancel it with:

heroku builds:cancel BUILD_ID -a YOUR_APP_NAME

Et voilà, this will force fail the build.

Note: you could also get the build id from the build log URL.

I might have found an answer to this problem, it seems to have been answered by Heroku in May. I'm assuming that by release phase they mean deploy:https://help.heroku.com/Z44Q4WW4/how-do-i-stop-a-release-phase

Release Phase processes are the same as any other Dyno in your formation, expect they run the codebase from the new release, instead of your current release.

To monitor your Release Phase processes as they execute, you can use the CLI command heroku ps -a YOUR_APP_NAME. as these are normal processes, you can use the ps:kill and ps:scale commands to stop the Release Phase from completing, which in turn, will prevent the latest release from completing.

I haven't tested this yet, but i will update with my exact commands when i have tested it out. If any one tests this out and can confirm, please feel free to update this answer.

I used the command

heroku builds:cancel -a <your_app_name>

and this worked for me