Spacebar won't stop preview

Hi all,

I've seen plenty of topics about this but haven't yet (unless I've missed it) spotted a solution to my issue.

Whenever I press spacebar to playback in AE it starts playing as you'd expect (slower on frames that it needs to render/cache as expected) but when I hit spacebar to stop it from previewing/caching nothing happens - it continues until it gets to the end of the timeline.

Please can someone help, this is driving me mad.

[edit] Oh god, please make it stop! I've been watching the same thing on loop for 10 minutes now!
Kind regards,

Chris