I have been reading the doc and searching but cannot seem to find a straight answer:

Can you cancel an already executing task? (as in the task has started, takes a while, and half way through it needs to be cancelled)

I found this from the doc at Celery FAQ

>>> result = add.apply_async(args=[2, 2], countdown=120)
>>> result.revoke()

But I am unclear if this will cancel queued tasks or if it will kill a running process on a worker. Thanks for any light you can shed!

revoke cancels the task execution. If a task is revoked, the workers ignore the task and do not execute it. If you don’t use persistent revokes your task can be executed after worker’s restart.

https://docs.celeryq.dev/en/stable/userguide/workers.html#worker-persistent-revokes

revoke has an terminate option which is False by default. If you need to kill the executing task you need to set terminate to True.

>>> from celery.task.control import revoke
>>> revoke(task_id, terminate=True)

https://docs.celeryq.dev/en/stable/userguide/workers.html#revoke-revoking-tasks

In Celery 3.1, the API of revoking tasks is changed.

According to the Celery FAQ, you should use result.revoke:

>>> result = add.apply_async(args=[2, 2], countdown=120)
>>> result.revoke()

or if you only have the task id:

>>> from proj.celery import app
>>> app.control.revoke(task_id)

@0x00mh’s answer is correct, however recent celery docs say that using the terminate option is “a last resort for administrators” because you may accidentally terminate another task which started executing in the meantime. Possibly a better solution is combining terminate=True with signal="SIGUSR1" (which causes the SoftTimeLimitExceeded exception to be raised in the task).

See the following options for tasks: time_limit, soft_time_limit (or you can set it for workers). If you want to control not only time of execution, then see expires argument of apply_async method.

In addition, unsatisfactory, there is another way(abort task) to stop the task, but there are many unreliability, more details, see:
http://docs.celeryproject.org/en/latest/reference/celery.contrib.abortable.html

Per the 5.2.3 documentation, the following command can be run:

    celery.control.revoke(task_id, terminate=True, signal="SIGKILL")

where
celery = Celery(app.name, broker=app.config['CELERY_BROKER_URL'])

Link to the doc: https://docs.celeryq.dev/en/stable/reference/celery.app.control.html?highlight=revoke#celery.app.control.Control.revoke

from celery.app import default_app

revoked = default_app.control.revoke(task_id, terminated=True, signal="SIGKILL")
print(revoked)

You define celery app with broker and backend something like :

from celery import Celery
celeryapp = Celery('app', broker=redis_uri, backend=redis_uri)

When you run send task it return unique id for task:

task_id = celeryapp.send_task('run.send_email', queue = "demo")

To revoke task you need celery app and task id:

celeryapp.control.revoke(task_id, terminate=True)