I wouldn't say unstoppable for me...but definitely finicky here. They've attempted to make preview playback and rendering separate from using the interface...so you can adjust things while it's playing (as opposed to the way it used to work, where the preview would stop if you clicked anywhere or hit a key). So I'm wondering if that behavior is over-riding stopping a preview?
I still prefer numpad+0 to kick off a preview, and it seems I have to hit it several times to stop it. (2 or 3)