If you want to stop clients from downloading packages, you will need to disable the respective policy/task, have the clients update their config, and then restart their sma service.
It would be nice if there was an emergency stop command you could send out that would simply tell the sma to cutoff the download, but there isnt.
If you want to stop ps's from downloading a package, you will need to remove the ps assignment from the package, have the ps update its config, and then restart its sma service.