cylc kill "foo*" kills all matching running tasks, but if any foo* are preparing they will go on to submit their jobs almost immediately.

This is not what users would expect. Preparing tasks are as good as running. If I issued the kill command 2 seconds later the jobs would have been submitted and then killed.

So, preparing tasks should get "killed" and set to submit-failed (and held, to prevent automatic submit-retries).

It's not clear if this will actually be easy to do. Several ideas discussed on Element...